Text about the skepticism concerning the external world. A first version was published in the Journal Abstracta. The final version will be published in the book Lines of Thought: Rethinking Philosophical Assumptions (Cambridge: Cambridge Scholars Publishing).
THE SKEPTICAL DEAL WITH OUR CONCEPT OF EXTERNAL REALITY
Philosophy unravels the knots in our thinking; hence its results must be simple, but its activity is as complicated as the knots that it unravels.
The so-called argument from ignorance concerning the external world is one of the most puzzling skeptical arguments ever conceived. In order to understand how it works, we must first examine some examples of general skeptical hypotheses about the external world. Here are a few:
1. I am dreaming the external world.
2. I am hallucinating an external world.
3. I am a soul being deceived by a malign genie, who makes me believe that I am living in this world, which in fact does not exist (the Cartesian version).
4. I am a brain in a vat, linked to a supercomputer that makes me believe I am living in a real world, while what I am really experiencing is a virtual reality program implemented in the computer (the main contemporary version).
Typical of such skeptical hypotheses is that it is at least logically possible that they are true, since it seems impossible to refute them decisively. Indeed, it seems that we are not even able to know that they are false.
Now, consider the trivial statement ‘I have two hands’, and the skeptical hypothesis ‘I am a brain in a vat’. Using them, we can already conceive the following instance of the argument from ignorance about the external world:
1. If I know that I have two hands, then I know that I am not a brain in a vat.
2. I don’t know whether I am not a brain in a vat.
3. Therefore: I do not know whether I have two hands (1, 2 MT).
Indeed, if I cannot know that I am not a brain in a vat, how can I know that I really have two hands?
Since the statement and the skeptical hypothesis can change, calling p any statement about the external world, even trivial ones like ‘I have two hands’, ‘This table exists’, ‘There are stones’…, calling K the operator for knowledge, and h any skeptical hypothesis, the general form of the argument can be represented in the following modus tollens:
1 Kp → K~h
3 \~Kp (1, 2 MT)
At first glance, this argument seems compelling: Since I cannot know that I am not a brain in a vat, it seems that I cannot know the reality of anything belonging to the external world.
Nonetheless, the argument from ignorance seems less convincing when we see that we can apply a modus ponens, building a converse anti-skeptical argument that could be called the argument from knowledge concerning the external world. This converse argument has the following logical form:
2 Kp → K~h
3 \K~h (1, 2 MP)
And here is an instantiation:
1. I know that I have two hands.
2. If I know that I have two hands, then I know that I am not a brain in a vat.
3. Therefore: I know that I am not a brain in a vat. (1, 2 MP)
Both arguments seem similarly compelling. Which is the right one? Who is able to pull the trigger first? The skeptic or the anti-skeptic?
In this paper, I give a negative answer to both these alternatives, for I think that both arguments are equivocal. I intend to justify this claim with the help of a long argument with three steps: in the first step, I will show that in all their stages both arguments involve attributions of external reality that can be made explicit. In the second, I will analyze the concept of external or objective reality, showing that it has at least two different senses, one belonging to everyday life, another belonging to the very unusual contexts of skeptical hypotheses and artificial, fictional realities. Finally, in the third step I will show that the attributions (or disattributions) of external reality slip from one of these senses to the other in the course of both arguments, which can be clearly shown after these attributions/disattributions are made fully explicit. These slides of sense make both arguments equivocal and therefore fallacious.
Making explicit the attributions of external reality in the arguments
Kant already noted that the category of reality (Dasein/Nichtdasein) is applied in all categorical judgments. The first thing to be considered is that attributions (or disattributions) of external, objective or concrete reality or existence are always considered in each stage of the arguments from ignorance and knowledge, although typically in an implicit way. Indeed, all the stages of the arguments depend on implicit considerations of attributions/disattributions of external reality. So, consider the argument from ignorance. What the argument claims is that since we lack knowledge of ~h, we are unable to know the reality (or existence) of the external world, and that because of this lack of knowledge of the reality of the external world, we are unable to know the reality of any state of affairs belonging to it. We are unable, therefore, to attribute external reality to anything stated by p.
In both arguments, what is meant by Kp is that I know the external reality or existence of the fact represented by p. Therefore, the conclusion ~Kp of the argument from ignorance amounts to the same thing as the conclusion that I do not know that I can attribute external reality or existence to the content of p. Although always given, such attributions of external reality involved in a trivial statement p remain more or less implicit in our normal speech. So, when p is the statement ‘This piece of chalk is real (or exists)’, the attribution of (external) reality is indeed explicit, for existence is the issue. However, when p is ‘I am holding a piece of chalk’, the attributions of reality remain implicit, although the statement can be made more explicit as ‘I am really holding an externally real piece of chalk’. Attributions of external reality usually remain implicit for a simple reason: since they are always involved, it is superfluous to spell them out. (The case is like that of statements: these are so commonplace that one does not need to make explicit the illocutionary act of stating by saying, ‘I state p’.)
A way to show the attributions of external reality implicitly involved in knowledge claims concerning the external world is to deny that we know the external reality of what is said in the utterances. It is blatant nonsense to say, ‘I know that this piece of chalk is real, but I do not know whether it is real or not’; and it is no less a nonsense to say, ‘I know that I am holding a piece of chalk, but I do not know whether it is (externally) real or not, and I do not know whether I am really holding anything’. It is therefore clear enough that since Kp is a statement about the external world, an attribution of external reality to its factual correlate must always be at least implicitly involved.
Once we have grasped this, argument (I), instantiating (A), can be restated in a way that makes explicit the assumptions concerning external reality:
1. If I know that I have two (externally) real hands, then I know that I am not a(n) (externally) real brain in a real vat.
2. I do not know whether I am not a(n) (externally) real brain in a real vat.
3. Therefore: I do not know whether I have two (externally) real hands. (1, 2 MT)
However, argument (II) instantiating (B) can also take a form that makes explicit the attributions of external reality:
1. I know that I have two (externally) real hands.
2. If I know that I have two (externally) real hands, then I know that I am not an (externally) real brain in a real vat.
3. Therefore: I know that I am not a(n) (externally) real brain in a real vat. (1, 2 MP)
These arguments only make explicit what is already assumed in (I) and (II), namely their concern with external reality. As promised, later on I will show that the attributions of external reality in both arguments have a different meaning in the premises than in the conclusions, which makes them equivocal. However, in order to achieve this end we need to make a sufficiently detailed preliminary examination of the meanings of expressions such as ‘external reality’ and ‘objective existence’.
Carnap’s semantic distinction and its limitations
In the search for an analysis of the concept of external or objective reality or existence in its association with skepticism concerning the external world, we may ask whether Rudolf Carnap’s famous distinction between external and internal questions of existence or reality could be of some help. His distinction applies to all domains of knowledge, but its application to what he calls the ‘world of things’ (the external world) is what interests us here. The usual questions about reality or existence concerning the external world are what he calls internal questions. In this case, he writes:
…to recognize something as a real thing or event means to succeed in incorporating it into the system of things at a particular space-time position so that it fits together with the other things recognized as real, according with the rules of the framework.
Therefore, when we ask ourselves whether the Statue of Liberty really exists, or whether there is really a Santa Claus, we are stating internal questions, the first of which is very successful in including a thing among other things belonging to the external world, while the second is not.
As Carnap also notes, philosophers may ask about the existence of the world of things (the thing-world) in themselves, about the reality of the external world as a whole. For him it would be misleading to ask whether our-external-world-as-a-whole really exists, understanding this as an internal question of existence. It would be to state a metaphysical question that is unverifiable and consequently senseless – for an internal question can be answered, and therefore stated, only when it is about things related to one another within the system, and never when it is about the system as a whole. For Carnap, a question about the existence of the world of things would only make sense when understood as an external question, which concerns merely our decision to use a linguistic framework for the world of things (the thing-language for the thing-world). This acceptance, however, is not the result of a cognitive decision, but rather of a pragmatic one, based on factors like the expedience, fruitfulness and efficacy of the framework.
Although Carnap’s distinction does not seem to be without a difference, it has its own flaws, already pointed out by philosophers such as Barry Stroud and P.F. Strawson. These thinkers have convincingly argued that the problem of the reality of the external world as a whole cannot be reduced to the mere status of a non-cognitive linguistic decision: We are in some way unavoidably led to accept the external world’s existence.
In order to understand that there is more to the question than the assumptions in Carnap’s distinction, consider the following statement showing a pervasive ambiguity in our attributions of reality to the external world:
(1) I know that the (external) world is real.
This statement is ambiguous. It could mean:
(1a) I know that our external world has reality,
but sometimes it seems that it can also mean that I know that this is the external world, or
(1b) I know that our world (contrary to skeptical hypotheses) is the ultimately real one.
The difference between (1a) and (1b) becomes clearer when we consider what truth-value we would give to each statement. Consider the negation of (1a):
(~1a) I do not know whether our external world has reality.
Surely, most of us would agree that (1a) is true, whereas (~1a) is false. I know, and we all know, that our external world has reality in the sense that it contains reality, that it is full of reality. This truth can only be denied metaphorically, as when a poet imagines fog-shrouded London as an “unreal city.”
But statement (1b) is a different case; it seems to be false. To see this more clearly consider its negation:
(~1b) I do not know whether our world (contrary to any skeptical hypothesis) is the ultimately real one.
Statement (~1b) seems to be true, because we feel that we do not have epistemic resources to reject the logical possibility that, unbeknownst to us, some skeptical hypothesis about the external world is true and that its reality is only virtual.
In contrast to (1b), (1a) cannot be shown to be false, even if it is true that I am a brain in a vat or a soul deceived by a malign genie. Even if a skeptical hypothesis were true, I would still be justified in thinking that the world I am experiencing is a perfectly real one, and not something like, for example, the limited world I experience when watching a movie or when asleep in the faint world of my dreams.
Our question now is: what kind of relationship could be found between Carnap’s distinction and the present distinction between two cases of attributions of reality? Initially, we have a strange feeling that sense (1a) of the statement ‘The (external) world is real’ concerns an internal question of reality, while sense (1b) concerns an external question of reality. However, since both statements concern the whole world of things, it is clear that, following Carnap, the attribution of reality in both of them should answer an external question of reality that would be established as the result of pragmatic decisions... In this case, however, why do we distinguish (1a) from (1b), regarding the first as true and the second as false? Why are we so prone to attribute cognitive status to (1a) but not to (1b)? In what follows, we will delineate an analysis of the concept of objective reality that makes possible an answer to these questions.
Introducing a new semantic distinction
My own strategy for analyzing different kinds of attributions of external reality was inspired by Wittgenstein’s later semantic reflections and methodological devices. I will assume two very plausible semantic insights derived from his writings. The first is the view that the meaning of an expression can be approximated from how it is used, and that a difference in the way of using or employing an expression corresponds to a difference in its sense or meaning. Attention to the praxis of our language shows that there are many nuances of meaning that we are seldom aware of. This lack of awareness, if not the only source of philosophical problems (as Wittgenstein sometimes seems to think), is at least a relevant source of philosophical misunderstanding, particularly in the case of what seem to be non-substantive riddles such as the skeptical one.
The second insight is his view that the criterial rules for the application of an expression are constitutive of its meaning. According to Wittgenstein, without criteria for its application an expression is devoid of meaning, and when we change these criteria, we change its meaning (its form of application, its way of use).
The first semantic insight is related to the second by the fact that when we speak about ways of use we are speaking about rules (or combinations of rules) determining the spatio-temporally located, episodic uses of expressions. Criterial rules (or combinations of rules) can be seen as rules that cognitively contribute to these episodic uses of expressions. Consequently, as a way to explain the meaning of conceptual expressions, we can perform a criterial analysis, making explicit the criteria for the application of the expressions.
Applying the first insight to what seems intuitively right in Carnap’s distinction between internal and external questions, it is clear that in the typical case when we ask or answer an internal question about existence, we are using the concept of external existence or reality in a way that differs from the way we use the concept of external existence or reality to ask or answer an external question. Consequently, we are applying the concept of external existence or reality in different senses. So, when we ask an internal question, such as whether the Statue of Liberty really exists, we are using the expression ‘really exists’ in a sense that is different from the sense it has when we ask whether the external world as a whole really exists in itself and not, for example, as a virtual reality produced by a supercomputer…
I will call the sense or kind of attribution of external reality usually linked with an internal question an inherent one, and the sense or kind of attribution of external reality that could be linked with an external question – concerning the reality of the world beyond the skeptical hypothesis – an adherent one.
There are some linguistic clues that confirm this distinction. First, a linguistic feature of the inherent sense is that the words ‘real’ or ‘exists’ can be replaced by the English word ‘actual’. Moreover, instead of saying that something is inherently real or exists inherently, we can in this sense also say that it possesses reality, that it has reality, or that it is full of reality: the piece of chalk that I am holding is actual, it has or possesses reality, it is informed by reality. On the other hand, something that is not real in the inherent sense, such as an imaginary piece of chalk, is also called non-actual, or non-existent, something that does not have reality or is empty of reality. However, the same contrast does not apply to the concept of adherent reality. When we attribute adherent reality to something (my hand, the world in which it is located…) we can say that it is actual, that it possesses adherent reality. However, when we negate the attribution of adherent reality, we cannot negate actuality, either the possession of reality or full reality. We cannot affirm that a world that is adherently non-real (like the world of the brain in a vat) and everything in it (like my illusory hand and the illusory chalk) lacks these properties. Such a world, although adherently unreal, is conceived as perfectly actual. Moreover, this world can be said to possess (inherent) reality, to be fully real in its experience. Indeed, it seems that actuality and the possession of reality are properties of inherent reality, not of adherent reality.
Although we have found some linguistic clues confirming our proposed distinction between inherent and adherent senses or attributions of reality, the distinction remains unclear. However, this distinction can be sharpened and more profoundly analyzed when it is considered in terms of criteria. In what follows, I will make what may be called a criterial analysis of conceptual expressions attributing external reality – expressions like ‘external reality (or existence)’ or ‘objective reality (or existence)’ – in order to distinguish more adequately the inherent from the adherent senses. I will begin with a search for criteria for inherent attributions of reality.
Inherent senses of our attributions of reality
Let us consider, in searching for criteria, the conceptual expressions used for the attribution of external or objective reality or existence in the supposedly inherent sense. The genetically primary use of these expressions seems to be when we ask whether things in the external world around us really do exist, since they are the first objects of our acquaintance. According to our understanding of Wittgenstein, we are entitled to suppose that the inherent sense of the conceptual expressions used for the attribution of external reality to the things around us is constituted by criterial rules for this attribution. Such rules would show us that only with the satisfaction of certain criteria of external reality would we be able to apply expressions like ‘is externally real’, ‘exists objectively’, ‘is actual’, ‘has concrete existence’ in the inherent sense. But can we find such criteria?
My claim is that criteria do indeed exist for the inherent sense of attributions of reality. They have been addressed by many influential thinkers. Thus, according to the representationalist Locke, our opinions of material objects are justified by properties associated with our ideas of senses, like their involuntary character, their orderly and coherent agreement reflecting law-governance, and other people’s awareness of them. According to the immaterialist Berkeley, ideas formed by the imagination are faint, indistinct and also entirely dependent on the will, while ideas perceived by the senses are vivid and clear and have no dependence on our will. For Hume too, impressions are seen as perceptions of real things when they enter the soul with the ‘most force and violence’, unlike the ‘faint images’ that we have in thinking and reasoning. For Kant, the conformity to law (Gesetzmäbigkeit) of all objects of experience is what defines the formal aspect of nature. For J. S. Mill the external (material) world consists of continuous or warranted possibilities of sensation, following from one another according to laws. According to Gottlob Frege, the main criterion of objectivity is interpersonal accessibility, followed by independence of the will, while the main criterion of reality is spatial and/or temporal location; hence, the realm of objective reality is for him made up of those things that are interpersonally accessible and spatially and/or temporally given. In a well-known paper, G. E. Moore summarizes the properties of external reality, stating that the real is something independent of the mind that is verifiable by others, continuously connected with other things, and having in this way certain causes, effects and accompaniments (I would say ‘following regularities’) with the highest degree of reality. Finally, a psychoanalyst such as Sigmund Freud suggested that a new-born child is animated by the pleasure principle, always seeking immediate gratification and unable to distinguish between the external and internal worlds. Only gradually does the child learn that the external world, unlike the world of her fantasy, does not obey her will, and this forces her to learn to postpone the gratification of her drives, and in this way she comes to replace the pleasure principle with a different one, namely, the reality principle.
Indeed, starting in childhood, we gradually learn to distinguish external reality from appearances by means of criteria such as the greatest intensity of sensation, independence of the will and interpersonal accessibility, and it seems to be a conceptual truth that world-states without any of these properties should be said to be unreal, non-real or non-existent in the usual sense. Although it was already argued that criteria like these are of no use, since none of them is sufficient, we still have the alternative of combining them, claiming that taken as a whole they are strong enough to be conclusive. Doing this in a non-systematic way, we can say that the things around us – using the word ‘thing’ in the widest sense, in order to include objects, properties, conditions, states of affairs, events, processes, etc. – are real when:
1. the sensory experience of them has the greatest intensity,
2. they remain independent of our will,
3. they are interpersonally checkable by anyone in the right position and their experience is usually co-sensorial (although this last requirement can vary from case to case).
4. they display regularities (external things obey regularities such as those imposed by natural laws, social norms, etc.).
These are what we may call the standard criteria of external reality. My contention is that we apply these criteria together all the time in order to be certain of the reality of the things around us. These four criteria together are what must usually be satisfied for a justified application of predicates like ‘…is externally real’, ‘…exists objectively’ in their primary inherent sense, namely, attributing reality to things belonging to the external world surrounding us. Moreover, taken together these criteria form what we could call a definitional criterion in the sense that once they are given, they warrant the attribution of inherent reality. In other words, the joint satisfaction of all these criteria is a sufficient condition for the attribution of external reality.
In order to exemplify, let us suppose that I hold a piece of chalk and say ‘The piece of chalk I am holding is real’, or simply ‘I am holding a piece of chalk’. Insofar as it is assured that criteria (1) to (4) are satisfied, I am entitled to think that the piece of chalk isn’t a figment of my imagination, but something externally real, objectively existent in the inherent sense. Indeed, in order to be true the claim ‘The piece of chalk I am holding is (externally) real’ must satisfy criterion (1), because the intensity of the sensations is maximal, unlike those in a dream. It must satisfy criterion (2), because the chalk is independent of my or your will (I cannot make it disappear like a mental image). It must satisfy criterion (3), because its experience can be the object of interpersonal verification, that is, current or previous experience convinces me that it can be recognized as the same by any other knowing subject (usually we cannot share a hallucination – collective hallucinations aren’t easily conceivable). Moreover, usually it is co-sensorially experienceable: I can not only see, but also touch and smell the piece of chalk. But this sub-condition cannot be generalized to all cases: the rainbow cannot be co-sensorially experienceable. Finally, a real piece of chalk must also satisfy criterion (4), because it must have the regularities of objects that comply with natural laws: the chalk can be used to write on a slate, one can break a piece of chalk. If dropped, a piece of chalk falls to the ground, while imaginary chalk might even float in the air, etc.
When we attribute reality to things in the world, it is because we assume that they satisfy these criteria, either directly to our senses (as in the case of opaque medium-sized dry goods) or indirectly by means of their association with possible phenomena (as in the case of the subatomic entities posited by science, like quarks, neutrinos or electromagnetic waves). If all these criteria are satisfied, the piece of chalk must of necessity be seen as inherently real. It is in this way that we succeed in incorporating a thing among other things in a spatio-temporal system of things according to the rules of the thing-language (in this case the criterial rules), as Carnap requires for his internal questions of existence.
Indirect satisfaction of the criteria of reality
There are many things that we find around us to which we ascribe reality, although they in no way meet the criteria of inherent reality in its default form. This is the case with entities discovered through scientific research, such as bacteria, viruses and DNA segments in biology, molecules in chemistry, and entities like the subatomic particles and forces postulated by physics. How is this possible? We can clarify the problem by means of an example. Suppose that a trail is observed in a cloud chamber and that we conclude that this trail was produced by the passage of a positron. Certainly, the trail meets the standard criteria of reality: it has maximum perceptual intensity, it is independent of the will, it is interpersonally checkable, etc. Therefore, we say that the trail is a real trail made by the passage of a positron. The positron, however, isn’t directly verifiable, because it is not itself visible and does not seem to satisfy anything like the standard criteria of reality. Even so, we are willing to say that in order to be the cause of the streak, the positron must exist objectively, it must be real. How is this possible?
The answer relates to the process of semantic extension, already mentioned by Aristotle in his account of focal non-homonymity. I take as an example his famous account of the semantic extensions of the word ‘health’. Supposedly, the predicate ‘healthy’ was primarily and originally assigned only to humans and animals, and later extended to food and exercise, which also came by convention to be called ‘healthy’, inasmuch as they are causal determinants of the health of living organisms. The principle of semantic extension applied to such cases states that: If A causes the property F of B, then we may be entitled to extend the assignment of F to A. In other words: if food and exercise have the property F of promoting health in human beings, then we are entitled to extend the assignment of F to food and exercise, even if the sense is not really the same, since it means ‘promoting health in humans and animals’.
Something similar can be said about the concept of external reality in order to explain the extension of this concept to things that cannot be experienced. In the external world, the property of the real existence of a given effect is in some way causally related to the source of this effect. Now, my claim is that the above clarified principle of semantic extension also applies to the case in which F is an attribution of external reality. This suggests that if effects are considered real, their causes may also be said to be real, even if they cannot be directly experienced and thus cannot be subjected to our standard criteria of reality. In other words: if certain things meet the standard conditions for external reality, so that we can attribute inherent reality to them, then we can also assign the inherent sense of reality to their causes, even though we cannot say that they satisfy those criteria. As a terminological matter, I propose we say that in this case the standard criteria of reality are secondarily satisfied, meaning by this that something indirectly satisfies the criteria of external reality when its effects satisfy them in the usual, direct way. Therefore, we can say that the cloud chamber was traversed by a real positron: it was real in the extended sense of being a cause of effects that meet the standard criteria for the inherent sense of the word ‘reality’.
It is important to note that the converse semantic principle is also true: If A is caused by the property F of B, then we may be entitled to extend the attribution of F to A. Thus we can say, for example, that a physically strong man is healthy, although we are applying the word to an effect of his health. Applied to the concept of reality, this principle supports the idea that if causes are real, their effects can also said to be real. Or: if causes satisfy the standard criteria for the inherent sense of reality, then we can say that their effects indirectly satisfy them. Thus, for example, if the movement of an iron bar magnet, which is real, produces the movement of electrons in a copper wire, the inferred electromagnetic energy generated by this motion can be considered real.
Again, considering the same example, we can note that the generated energy, in turn, also has an effect. Thus, for example, it can be measured with a galvanometer. The movements of the galvanometer’s pointer are real according to the standard criteria of reality. However, in applying these criteria we see that we have two ways to extend the concept of reality to electromagnetic energy: we assign it a reality based on a semantic double transfer from the effect to the cause and from the cause to the effect. Therefore, we conclude that we can regard things as indirectly satisfying the standard criteria of inherent reality if they are contained in a network of effects and causes that do satisfy those criteria.
I think that these considerations explain our inclination and even our semantic right to attribute reality to things that we cannot experience with our unaided senses, as well as to say that we can experience them indirectly. Moreover, I believe that this also inclines us to furnish the appropriate rationale for scientific realism, a view that I share (anti-realists could try to reject such an extension of fulfillment of criteria as a misappropriation of the concept of reality).
We have considered methods for extending the concept of reality to the realm of things that can be said to be experienced only indirectly. Now we need to give a justification for this extension. We want to know why we are entitled to apply the concept of reality to things that are causally or constitutively related to things that are directly experienced. In my view, the reason is that the boundaries in the inherent sense of reality between things that satisfy the criteria directly and those that satisfy them only indirectly are not categorical and definitive. They are contingent and variable, since to a considerable extent they depend on the contingent nature of the subject’s sensory organs. To highlight this point, imagine that our senses were different than they are. Suppose that we were aliens with visual organs capable of a very high level of resolution, so that we could actually see bacteria with the unaided eye. Or suppose that our brain contained sensors that made us aware of electromagnetic fields or that we were so constituted that we had a kind of innate Geiger counter and could detect dangerous levels of radiation... In such cases, the domain of application of our standard criteria of external reality would be larger. This domain would extend to many things we currently regard as real because they appear to satisfy secondary criteria of reality. This merely circumstantial character of what should be regarded as the standard way to satisfy the criteria for the inherent sense of reality gives a rationale for the idea that we are entitled to extend our attributions of reality to things that we cannot experience directly with our unaided senses. It shows that the difference between the primary and secondary satisfaction of criteria of reality is largely contingent and arbitrary. Consequently, there is no good reason why we should not extend the domain of the real beyond what we can directly experience with our unaided senses.
Proof of the external world
We already saw that our attribution of reality can be extended to postulated entities that cannot be directly experienced. However, there are much more commonplace spatio-temporal extensions of these attributions. As discussed above, we apply our concept of external reality originally and primarily to (A) those objects, properties, conditions, states of affairs, events, processes … that surround us and presently satisfy (directly or indirectly) our standard criteria of external reality (e.g., respectively this computer monitor and the electrical energy that illuminates it). But this is not all. We also apply the concept of external reality to (B): many other things that are not immediately experienced, but which we have good inductive reasons to suppose would, under suitable conditions, satisfy our standard criteria, and consequently can also be regarded as real. This is the case with (B1): everything we have ever experienced that is at present too remote or inaccessible to us to be (directly or indirectly) experienced. This is also the case with (B2): many things we know to satisfy the criteria only via the testimony of others. And this is surely also the case with (B3): many things that certainly must exist (we are assured), even though they have never been – and probably never will be – experienced by anyone (the so-called openness of the world), because we are constantly experiencing other things that we would never have expected. My claim is that based on (B1), (B2) and (B3) we can inductively infer (beginning with the successive experience of things around us that directly or indirectly satisfy the standard criteria of inherent reality) that there is presently a whole world of things around and beyond us that satisfy these criteria of reality in the sense of being confirmed as able to satisfy them experientially under the appropriate circumstances, although they are not presently being experienced. This is why it is not wrong to use concept-words like ‘real’ or ‘exists’ in order to claim that our external world as a whole is objectively real or exists externally, insofar as by ‘our external world’ we mean something like ‘the sum of all the things that we think satisfy our standard criteria of external reality’. This is an extension of the inherent sense of our attributions of external reality that Carnap failed to consider when he proposed his distinction between internal and external questions of existence.
Accepting this kind of reasoning, I will call all attributions of reality going beyond the present experience of our surroundings, including indirect experiences of things, extended inherent senses (uses, ways of application) of our conceptual expressions for external reality, in contrast to what we might call the primary inherent sense, which is given by the direct application of the standard criteria of reality. When the inherent sense of the word ‘reality’ is extended to the maximal limits of its application, we apply it to the external world as a whole.
Now, using the word ‘experience’ in a broader sense – which includes not only the awareness of the satisfaction of primary criteria, but also of the satisfaction of the secondary criteria of external reality – and having in mind only the inherent sense of external reality or existence, we could formulate a rough reconstruction of the reasoning that leads to the commonsensical conclusion that our external world as a whole really exists, that it is real in the inherent sense, as follows:
1. Many things that are presently experienced satisfy (as I have stated, primarily or secondarily…) the criteria of external reality (our bodies, the external things around us…).
2. Most of the things we have experienced in the past have been experienced more than once and have satisfied the criteria of external reality whenever they were experienced.
3. (inductively from 2) There are things that have been experienced in the past and, although they are not being experienced right now, (are still able to) satisfy the criteria of external reality.
4. We are constantly experiencing new things around us that satisfy the criteria of external reality.
5. (inductively from 4) There must be as yet un-experienced things that satisfy the criteria of external reality.
6. Testimony constitutes a reliable way to achieve knowledge.
7. There is much testimony for things that satisfy the criteria of external reality.
8. (deductively, from 6 and 7) There must be many things that we have not personally experienced that satisfy the criteria of external reality, this being known via testimony.
9. (deductively, from 1, 3, 5 and 8) There is a totality of things, some of which are (A) presently experienced things satisfying our criteria of external reality, some of which are (B1) things not presently being experienced that we know satisfy our criteria of external reality, since they have satisfied these criteria in the past, some of which are (B2) un-experienced things that satisfy the criteria of external reality via testimony, and some of which are (B3) things that are still unknown, but are able to satisfy our criteria of external reality, for we are constantly experiencing new things satisfying these criteria.
10. What we mean by the idea of our external world as a whole is the totality of things, including those that are (A), (B1), (B2), or (B3).
11. (deductively, from 9 and 10) Our external world as a whole satisfies the criteria of external reality.
12. Whatever satisfies the criteria of external reality is (inherently) real.
13. (deductively, from 11 and 12) Consequently, our external world as a whole is (inherently) real.
In my view, this is the true proof of the external world, the proof whose absence was identified by Kant as the scandal of philosophy. Even if intended as a rough approximation, this argument is plausible enough for our purposes, for it is able to capture what is meant by the plain man when he makes statements that appear philosophically naïve, such as ‘It is obvious that the external world (as a whole) exists’ or ‘Only a madman would doubt the reality of our external world’. They intend to say that we have a good inferential basis to believe that the whole world, as a sum of its presently experienced, already experienced, and still un-experienced constituents, satisfies our standard criteria of external reality, and therefore actually exists. Indeed, even Stone Age man may have engaged in such reasoning in a non-reflexive way, since he could already judge, we surmise, that the world is real. However, a comprehensive reasoning like this remains tacit, unless the philosophical scrutiny reveals it to our consciousness.
The inherent senses of the concept of external reality are familiar and non-problematic. The existence of an extended inherent sense of reality can explain why we believe in the truth of statement (1a), telling us that we know that our external world as a whole has reality, for (1a) is the broadest formulation of the extensions of the inherent sense. It also explains why we somehow feel that (1a) should answer an internal and not an external question of existence. Moreover, the existence of the extended inherent attribution of reality shows that Carnap was mistaken when he supposed that one cannot meaningfully pose something like an internal question about the reality of our world as a whole, for this question would be unverifiable and metaphysical. He believed this because with the internal question he had in mind only the primary inherent sense of our attributions of external reality, along with some near extensions of it, without foreseeing the possibility of its inductive generalization to what we mean with words like ‘world’ and ‘universe’. However, we have already seen how, based on an inductive process that begins with the satisfaction of the standard criteria of reality, we may arrive at justified knowledge that the whole external world referred by us is real.
Adherent attributions of reality
Let us suppose that under the influence of a hallucinogenic drug for some hours I experience a perfect illusion of the world as it was in China in the time of Kublai Kahn. After the effects wear off, I can say to myself: ‘This was a world of my imagination, not the real one’, for I have good reason to think so. However, in this case I am not disattributing reality in the inherent sense, because all the standard criteria of reality have been sufficiently satisfied. In the case of this hallucinatory world, I suggest, I would be disattributing reality in the already named adherent sense of the word, a sense that can also be considered in skeptical contexts.
What are the criteria for this adherent kind of attribution of reality? We can explore this point using skeptical thought-experiments. For instance, I can imagine that one morning I could wake up in a completely strange environment, in a different body, surrounded by strange, alien creatures. They explain to me that until this point I was not living in the real world. For all of my previous life, I was only a brain in a vat, monitored by a supercomputer that simulated my external reality. They say that this is a conventional pedagogical procedure to foster mental diversity on the planet Omega, where during its development each new brain lives according to a different program, which in my case happened to be that of a ‘Philosophy Lecturer on Earth.’ But now, they further explain, my brain has been implanted in a real body, and I will henceforth live in the truly real world. Since all my further experiences turn out to be in full agreement with their explanations, I gradually decide that they were telling the truth, that the world I had previously experienced was not the real world, but only a virtual one.
It is important to see that I can find criteria leading me to this adherent disattribution of reality. However, they have nothing to do with a straightforward application of the standard criteria for any inherent sense of external reality! For the highest intensity of experience, the independence of the will, the co-sensoriality, the possibility of interpersonal access… were all already available to me when I was living my life as a brain in a vat on the earth, as much as they now are on the planet Omega. I can even say that my world, when I was just a brain in a vat, was actual, it had reality, in the primary and extended inherent senses, neither more nor less so than the world presently accessible to me in the here and now.
We have already discussed criterial configurations indicating that the past was adherently unreal. But it is just as easy to imagine the criteria working in the present or in the future. We can easily imagine criteria with which to decide whether or not the world in which one will live or is now living is the (adherently) real one. Let us suppose that under the legal code of planet Omega convicted murderers are not executed, but instead sentenced to spend the remainder of their lives as brains in vats. After being sentenced, a convicted murderer is anesthetized and his brain is removed and immersed in a vat provided with life-sustaining conditions. There he can continue to live a perfectly normal criminal life, even if disturbingly aware that he is experiencing only a virtual world where all reality is simulated by a supercomputer. He will live in a world that is perfectly real (actual) in the inherent sense, but which he knows is and will remain virtual, one that he knows is unreal or non-real in the adherent sense. (This should not sound so strange: during a nightmare we can semiconsciously, based on our self-awareness and memories, reassure ourselves that we are only dreaming…) As well here we may have criteria for the adherent non-reality of one world relative to another, insofar as we can be aware of the fabricated, artificial nature of one of the realities. It is in this relative sense that we can know that one of these worlds is not real.
It may appear that attributions/disattributions of adherent reality also concern only the whole world. However, the concept of adherent reality can equally be applied to specific parts of the world. Suppose, considering a common experience with virtual reality, that a person uses a special glove to close her hand around a holographic projection of a cup of tea. Inherent criteria like intensity, co-sensoriality, or even interpersonal access, might be satisfied. In this way, the holographic projection acquires some inherent reality... However, the fact that the person knows that this is an experiment going on under the particular conditions surrounding it serves as a criterion to convince her that the cup of tea she is holding is adherently unreal relative to the external world as we know it. In this way, the adherent reality of the fact referred by a statement p (e.g. ‘I have two hands’) can also be separately contested.
Criterial configurations for adherent reality
Now, how could we achieve the knowledge that some things are adherently unreal? How can I know, for example, that the world on the planet Omega is the real one and not an earlier world on the Earth? My reasoning could be formulated as follows:
5. All my most recent experiences are of the new inherent reality of the planet Omega (my new body, the bizarre creatures surrounding me, the unimaginably advanced technology…)
6. I still remember my experiences of the inherent reality of the very different world of Earth.
7. I know that standard inherent criteria of external reality can be satisfied in a fictitious environment (ex: co-sensoriality and interpersonal experience in dreams, or intensity and regularity in experiments producing a fictional reality).
8. I have heard reasonable explanations for the change, compatible with the hypothesis of a fictitious world (my earlier experiences were produced by a supercomputer, in compliance with pedagogical aims…)
9. These explanations are accompanied by evidence (I see the supercomputer and other brains in vats…)
10. (From 1 to 5) I conclude that the world of the Earth, contrasted with the world of Omega, is not the real one in the adherent sense, since its inherent reality was a manufactured by-product of the relatively adherent reality of the world of Omega.
What this argument shows is that I reached my conclusion that my previous world of the Earth was not the real one by means of a criterial configuration that has nothing to do with the standard criteria of inherent reality. The adherent criteria of external reality form a coherent configuration, which in the case above consists in the coherence of the new information with my awareness of my new and old experiences and knowledge. Moreover, when we consider the other cases in which adherent reality is produced, we see a similar coherence model with a variable criterial configuration. Consider my virtual experience in the China of Kublai Kahn: since I remember taking a hallucinogenic drug, and since my world before and after the experience remains the same, I can infer that the world I temporarily experienced was a mere drug-induced fantasy. Consider also the experience of virtual reality with the holographic projection of a cup of tea. Because of the available information and the context of the experience, the subject of the experience knows that the cup of tea, although inherently quasi-real, is adherently non-real. Important is above all that in each of these cases, the old world of the brain in a vat, the intermediate world of Kublai Kahn’s China, and the particular virtual reality of the holographic projection of the cup of tea, are only adherently unreal relative to what we accept to be an adherently real world.
We see now that the criteria used to identify an adherent reality must be completely different from the standard criteria of reality used to identify inherent reality. However, there is one link between adherent and inherent criteria of reality. This is that the criteria of reality in the adherent sense are used to decide between two conflicting (inherent) realities that already satisfy the inherent criteria of reality, distinguishing one of them as an illusory by-product of the other.
Contrary to the skeptical hypothesis, we could object that such criterial knowledge that the external world or even parts of it are adherently real or unreal is rather feeble. It could be, for example, that the new world of Omega in our first example was only another figment of reality, just as the first one… with a different program, called ‘Being awakened from a life as a brain in a vat’, was implemented in place of the old ‘Philosophy Lecturer on Earth’ program. It is also conceivable that my past life up until this event was lived in the real world, and my brain was removed from my head by aliens and then placed in a vat of liquid, where the new program ‘Being awakened from a life as a brain in a vat’ was implemented... It is even conceivable that the person in the case of the experiment with virtual reality was actually a brain in a vat and that the supposed holographic projection of a cup of tea was in fact the only real thing she has access to, with the help of a robotic arm operated from outside her fictional world! In conclusion: the proposed relative and variable criterial configurations for adherent reality are like shifting sand. Diverging from the cases of the standard criteria of inherent reality, which are non-defeasible when in fact given, the criteria for adherent reality are always non-defeasible, even when in fact given, which means that in the end we cannot really know whether any particular world is the real one...
My answer is that this objection demands an unnecessarily absolute sense of adherent reality, when in fact we have only a relative sense. In order to qualify this answer, we need to distinguish between two senses of the adherent attributions of reality that at first sight seem to be possible:
(a) a relative sense (considered in our examples of created realities).
(b) a non-relative or ultimate or absolute sense.
In what follows, I will show that sense (a) is workable, that there is a relative sense of adherent reality, while (b) is in fact an illusory sense of adherent reality.
We have already analyzed some examples in which the relative sense (a) of adherent reality was employed. The notion of a relative sense of a word is well known. The sense of a word is relative when it is arrived at by contrast within a context. The word ‘small’, for example, has a relative sense: a baby elephant is small relative to an adult elephant, but large relative to a mouse. The same is the case with the concept of external reality as conceived in skeptical thought-experiments and in other cases. Its sense is relative, acquired only within the context created by a skeptical hypothesis that has been shown to be true in the light of the given evidence, even considering that such evidence can always be challenged by new evidence. I maintain that this relative sense of our adherent attributions of reality, within its narrow limits, is legitimate, since we can conceive of coherent criterial configurations for it, as we did in the last section.
Consider now (b), the supposed non-relative or ultimate or absolute sense of our adherent kinds of reality attribution. It should answer the question of whether our world is ultimately the real one, that is, real in a non-defeasible way, real beyond any possibility of doubt arising from skeptical hypotheses. It seems clear that such criteria for ultimate reality cannot be truly available to us. We cannot conceive of them.
We come to the conclusion that the answer to the objection that we cannot really know if a world or any fact in the world is adherently real is in the affirmative only if we understand the concept of adherent reality in the sense of (b), that is as regarding ultimate adherent reality, for the criteria of adherent reality are always defeasible. However, the answer to this objection is in the negative when we understand the concept of adherent reality in the sense of (a), that is, as the relative adherent reality of a world or of a fact in it, weighted against the background of adequate information.
Reflection on this issue shows why we believe in the falsehood of statement (1b), which asserts that I know (without skeptical contrast) that our world is the ultimately real one. We cannot possibly know that much, because we lack criteria for knowing that the external world is ultimately real or that any skeptical hypotheses are definitely false or, when we have evidence for their truth, that it could not be refuted by other hypotheses that could also be refuted, and so on ad infinitum. From this we may conclude that we are also unable to know that parts of our external world are ultimately or absolutely real in the adherent sense – a conclusion that extends to the fact referred to by any statement p. Indeed, we cannot know whether our hands are ultimately real.
Because of his views, Carnap would be compelled to say that (1b) is true. He would have to say that in fact we attribute ultimate reality in the adherent sense to our external world as the result of a posit, of a pragmatic decision based on grounds like the expedience, fruitfulness and efficiency of a linguistic framework. However, as Barry Stroud has convincingly argued, not only does there seem to be no valid alternative to the thing-language, but it also does not seem to make any sense to claim that the external world’s existence depends on our decision to adopt a particular linguistic framework. In my view, Carnap reached this conclusion because he confused the extended inherent attribution of reality to the thing-world, which is non-relative, with the relative adherent attribution of reality, taking the second for the first. The result is the attempted fusion of both concepts in the non-relative positing of the reality of a world as something pragmatically justified.
It also seems that we are in fact unable to know the ultimate adherent reality or unreality of an external world. But we should not feel threatened by this conclusion, since it turns out to be harmless when we reflect that we do not have any criteria for knowledge here, and expressions without criteria are devoid of meaning. If we accept this, then the statement:
The external world is ultimately real (in the adherent sense).
is as devoid of sense as the statements ‘The whole world (with everything in it) doubled in size last night (in an undetectable manner)’ or ‘My brother will die the day after tomorrow’, which being unverifiable are like the useless loose wheel in the machine, to use Wittgenstein’s metaphor. Moreover, the following statement is true:
We do not know whether the external world is (ultimately) real.
This is like the statements: ‘We do not know whether the whole world doubled its size last night in an undetectable manner’ and ‘I do not know whether my brother will die the day after tomorrow’, which are likewise true. While true, both statements are vacuous enough to be trivial. Our world matters to us because of its quality of being inherently real (intense to the greatest degree, independent of the will, public, structured...) and not because it is an adherently real world in the ultimate sense, for we will never be in the position to know this.
How can the question of whether our external world is adherently real in a non-relative, ultimate or absolute sense not seem devoid of meaning? In my view, all we have here is an illusory impression of meaning that arises from confusing the adherent and the extended inherent senses of our attributions of reality to the whole world. In the extended inherent sense, it is perfectly correct to say that we know our external world is real, and this can be interpreted as an attribution of adherent reality. However, we are tempted to fuse the relative sense of the adherent attribution of reality to our world – only possible against the background of accepting or refuting skeptical hypotheses – with the extended inherent attribution of reality to our world. We may suppose that in this way we can arrive at an ultimate sense of these attributions that is devoid of criteria, but is what is at stake when we ask whether our external world (or some part of it) is the real one in the adherent sense. That is: our lack of awareness of these fine semantic distinctions is what leads us to see the absolute or ultimate sense of attributions of reality as though it were something meaningful, when it is in fact a fata morgana.
Recapitulating what we have learned so far
The result of our investigation is that we have found certain uses or senses of the expression ‘external reality’ that must be distinguished:
(a) The inherent senses or attributions of external reality, based on the standard criteria of reality (1) to (5). These inherent senses form a scale that begins with the primary inherent sense and continues with various degrees of extended adherent sense, culminating in its application to the whole world. These senses can be clarified as follows:
(a1) The original inherent sense of the concept-words for external reality. This sense is constituted by the application of criterial rules for the satisfaction of the standard criteria presently given to us, when we acknowledge the reality of things around us. (e.g. ‘My hands, this paper, this table, are real’.)
(a2) The extended inherent sense of concept-words for external reality. This sense inductively extends the application of standard criteria of reality to what is not presently or primarily experienced. In this sense, we can say that atoms or physical forces are real, we can say that many external things that we have experienced in the past are real, etc. If we consider all that can be said to be real in the original and extended senses of our concept-words for external reality, we arrive at the concept of the reality e world as a whole. The plain man is appealing to this sense when he claims: ‘Of course the external world is real; were it not real, it would not be our external world’. (When this sense is intended, it also answers affirmatively an internal question of existence applied to the whole thing-world, pace Carnap.)
(b) The adherent senses of concept-words for external reality. These senses can arise when we need to distinguish the pseudo-reality of dreams, hallucinations, artificial reality and the words considered in skeptical scenarios. In such cases, we can say that a world w1 is unreal because it is a by-product of a world w2, or that parts of w1 are unreal because they are products of a virtual reality produced by w1 itself… The important point about the adherent senses of the concept-word ‘reality’ is that they are relative to the contrasting contexts given by skeptical hypotheses, hallucinations and ways of producing fictional realities. The insistence on believing that there is an adherent sense of reality in the ultimate or absolute, non-relative sense, leads us astray. Considered in this way, the meaningfulness of questions concerning ultimate adherent reality is an illusion.
We are now prepared to advance our objections against the skeptical argument. In what follows, I will show that the skeptic is able to infer that we do not know the adherent reality of any trivial proposition p in an absolute sense. We have already seen that although this is true it is trivial, and the skeptic wants something more. What he wishes to deduce is that we do not know any world or fact in the world in its inherent reality, namely, in its actuality, its independence, its publicness. However, his attempt is doomed to failure, since the word ‘reality’ is used in its adherent sense in the premises and in its inherent sense in the conclusion, configuring an equivocal and therefore fallacious argument.
Refuting the Argument from Ignorance
Now we come to the last stage of our argument, which consists in applying the semantic distinctions between different kinds of attributions of reality to the skeptical and anti-skeptical arguments. Consider first the expanded form of the argument from ignorance about the external world:
1. If I know that I have two real hands, then I know that I am not in reality a brain in a vat.
2. I don’t know whether I am not in reality a brain in a vat.
3. Therefore: I don’t know whether I have two real hands. (1, 2 MT)
At first sight, this more explicit form of the argument is also flawless. However, it is not difficult to show that in this form – and consequently as well in its original form – the argument cannot stand up to scrutiny. This can be done by making explicit the kinds of attributions of reality it makes. By doing this, we obtain two direct ways of interpreting the attributions of reality that seem to make sense, a weaker and a stronger one. Only the second is usually intended by skeptics.
(a) Under the weak interpretation of the attributions of reality in the argument, the skeptic tries to convince us that we cannot know that all the knowledge we think we have of things belonging to the external world is not part of a universal illusion. In this case, all the attributions of reality in the argument would be understood as belonging to the adherent sense, as shown by the following formulation:
1. If I know that I have two (adherently) real hands, then I know that I am not in (adherent) reality a brain in a vat.
2. I do not know whether I am in (adherent) reality a brain in a vat.
3. Therefore: I do not know whether I have two (adherently) real hands.
Premises 1 and 2 are contextually understood as involving adherent attributions of reality, which makes them true, producing what I think is a sound argument (which would not be the case if the attributions of reality they make were all inherent). Indeed, it is true that ultimately I cannot know that I am not a brain in a vat in a non-relative adherent sense of reality, and following from this I do not know whether anything in the world is ultimately real in the adherent sense, including that I have two adherently real hands. Nonetheless, the conclusion that we cannot know the ultimate adherent reality of those things is entirely harmless and unsurprising. As we have already seen, to deny knowledge of the ultimate adherent reality of our world (and consequently of its parts) amounts to the same thing as to deny that we can know that the whole world doubled in size last night without this being in any way detectable or to deny that I can know that my brother died the day after tomorrow. Lacking criteria for their truth, these statements are devoid of sense, and to deny that we have knowledge of what they state is to say something true, namely, to reject knowledge of something that lacks any content. Hence, the impression that we are losing something important is false, arising from the confusion with the special case of relative attributions of adherent reality, which can only be made against the improbable background of a skeptical scenario that has been shown to be true. Moreover, in this weak interpretation, our knowledge of the inherent reality of things and states of affairs constituting the world, of their concreteness, which is important for our lives, remains unchallenged. This paltry conclusion cannot be what the skeptic means.
(b) It is clear that the skeptic is trying to convey a stronger understanding of the argument. This stronger interpretation is one that naturally suggests itself to us, since each sentence in it is understood in the most natural way. The conclusion is somehow unexpected, and we are attracted, out of curiosity, to what is new and non-trivial. Thus, skeptics understand the first two premises as sentences concerning adherent reality, since this is the natural way to understand them, for they are formulated in a skeptical context. But he understands the third sentence, the conclusion, as concerning the inherent reality of the referents of p, which is also the most natural and non-trivial way of understanding this sentence, since p is usually asserted in ordinary and non-skeptical contexts. As a result, the instantiation of the argument from ignorance will be as follows:
1. If I know that I have two (adherently) real hands, then I know that I am not in (adherent) reality a brain in an (adherently) real vat.
2. I don’t know whether I am not in (adherent) reality a brain in an (adherently) real vat.
3. Therefore: I don’t know whether I have two (inherently) real hands. (1, 2 MT)
This is the most compelling understanding of (I-a). However, the argument is obviously equivocal, because the kind of attribution of reality in the premises is different from the kind of attribution of reality in its conclusion. Here we are led to inadvertently slide from the understanding of the words ‘real’ and ‘reality’ in the adherent sense in the first and second premises to an understanding of the word ‘real’ in the inherent sense in the conclusion, which makes the application of the modus tollens impossible.
Taking advantage of our lack of awareness of the two senses of reality involved, the skeptic is confusing us with an argument in which the suggested conclusion is that we cannot know the reality of p in the usual inherent sense, or, in the case of our example, that I cannot know that I have two inherently real hands. He is suggesting that we do not know any state of affairs in our world as a concrete reality, that we cannot know them as having the maximal perceptual intensity, independence of the will, truly interpersonal access, etc., and that the plain man I wrong when he claims to know that he has two real hands or that our world really exists! The feeling of awkwardness produced by the argument from ignorance arises from this suggestion.
Finally, using ‘ir’ to indicate the inherent attribution of external reality in the statement and ‘ar’ to indicate the adherent attribution of external reality, the stronger form of the argument from ignorance can be expressed symbolically as follows:
1 Kpar → K~har
3 ~Kpir (1, 2 MT)
The unavoidable conclusion of our argument is that the most incisive form of skeptical argument about the external world is either sound, but harmless and trivial, or surprising and non-trivial, but invalid. It rests on a subtle fallacy of equivocation that falls apart when subjected to a sufficiently careful semantic analysis of what is involved in the use of our words.
Refuting the argument from knowledge
Now we will turn to the argument from knowledge, according to which because we know p, we know that h is false. Here too, we can find both a weak and a strong interpretation of the argument. The most natural and compelling interpretation of the argument from knowledge is the stronger. According to this interpretation, the argument makes explicit kinds of attributions of reality that seem reasonable in context and imply the truth of the premises. The assumptions in argument (II), as intended by the ‘dogmatic’ anti-skeptical philosopher, can be made sufficiently explicit as follows:
1. I know that I have two (inherently) real hands.
2. If I know that I have two (adherently) real hands, then I know that I am not in (adherent) reality a brain in an (adherently) real vat.
3. Therefore: I know that I am not in (adherent) reality a brain in an (adherently) real vat. (1, 2 MP)
This is the understanding of the argument that imposes itself to us, as far as each sentence in it is contextually understood in the most natural way. However, even though the two premises are true, I can’t apply the modus ponens to state the conclusion that I know it is ultimately true that I am not a brain in a vat, since the attribution of reality in the first premise can only be inherent.
Here, too, we can state the form of the argument symbolically in a way that shows that it is equivocal and thus fallacious:
2. Kpar → K~har
3. K~har (1, 2 MP)
Before concluding this section, I wish to comment on G. E. Moore’s attempt to prove the existence of the external world. Although this argument was conceived to refute idealism and not skepticism, I believe it can be fairly understood as a variation of the argument from knowledge interpreted in a weaker form. Consider his own words:
I can prove now, for instance, that two human hands exist. How? By holding up my two hands and saying, as I make a certain gesture with the right hand, ‘Here is one hand’, and adding, as I make a certain gesture with the left, ‘And here is another’. And if, by doing this, I have proved ipso facto the existence of external things, you will all see that I can also do it now in a number of other ways: there is no need to multiply examples.
explicitly states that he does not intend to refute the skeptics, but only to
prove the real existence of the external world, we can summarize what he
intends as: Moore
1 I know that I have two inherently real hands.
2 If I know that I have two inherently real hands, I know that there is an inherent reality around me (at least concerning my hands).
3 Therefore: I know that there is an inherent reality around me. (1, 2 MP)
There is nothing wrong with this argument, which hints in the direction of the proof of the external world already presented in this paper. Taken in this way, Moore’s argument has some force against the idealist, as he originally intended, insofar as the idealist (like the skeptic) is trying to build on the notion that our world is made from the stuff that dreams are made of. However, this argument would be too weak if used to prove the falsity of any skeptical hypothesis, since these hypotheses are concerned with reality in the adherent sense.
I conclude that both the argument from knowledge and the argument from ignorance, strictly considered, are misguided attempts to prove something that cannot be proved. We can know neither so much nor so little. This is why the skeptical and the anti-skeptical problems about the reality of the external world can be completely dissolved, I believe, as subtle forms of linguistic-conceptual philosophical bewilderment.
Argument from Ignorance Concerning our Past Reality
A similar kind of argument from ignorance can be applied to limited forms of skepticism, such as that about the past. Consider Bertrand Russell’s remark that it is possible that our whole world including ourselves with all our memories, was created only five minutes ago. Can we know that this hypothesis is false? Apparently not. In this case, how can we know whether anything was the case before those five minutes? The argument from ignorance concerning the past can be instantiated as follows:
1 If I know that the French Revolution occurred in 1789, then I know that the world existed before five minutes ago.
2 I do not know whether the world existed before the year 2000.
3 Therefore: I do not know whether the French Revolution occurred in 1789. (1, 2 MT)
Our answer to this skeptical argument resembles our previous one. We need to distinguish between two senses of our attributions of reality to a past occurrence. The first is an inherent sense, which depends on the insertion of our past occurrence in our historical framework. In this sense, I can claim to know that the French Revolution really did occur in 1789: there is enough documentary evidence available that shows us the inherent reality of this past historical occurrence. What makes possible this claim is once again an extended application of our standard criteria for external reality, this time applied to the past. The criteria for the inherent reality of historical occurrences are evidence of memory, testimony, documentary and physical historical evidence, etc. But as such they are secondary criteria, functioning as indirect ways to warrant that the past was real in the inherent sense, that is, that the past occurrence would satisfy our standard inherent criteria of reality (like the highest intensity of experience, independence of will, intersubjectivity…) for observers like us placed in the past. Indeed, saying that the French Revolution occurred in 1789 amounts to the same as claiming that the historical evidence shows that if observers like ourselves were living at the right time and place, they could have explained in more direct ways the satisfaction of the usual criteria for external reality concerning the events constituting the French Revolution.
However, there is another sense of real past existence that is adherent. This sense is external to our historical framework and could only arise in the context of a skeptical hypothesis about the past. Again, we can conceive of both a relative and an absolute sense of external adherent reality. To give examples of the relative sense we can, e.g., imagine being visited by God (or by aliens…), who is (or are) able to give us enough evidence that our world didn’t exist before the year 2000, that all our memories before this date are false, and that all the relevant historical evidence is a forgery. In this adherent attribution of reality, we cannot really know either that the world existed before the year 2000 or that the French Revolution really occurred in 1789. Indeed, in the adherent sense there is no way to obtain knowledge of the ultimate reality of the past, except a relative one. In this case we have unexpected evidence for the truth of the skeptical hypothesis, which makes the occurrences of the last five minutes the (adherently) real ones.
The clue to answering the skeptics is the same here: the skeptical argument is an attempt to confuse us, slipping equivocally from our ignorance of the adherent reality of our past to ignorance of the inherent reality of historical events as a conclusion. What follows is the most reasonable interpretation of the argument (because it seems sound and non-trivial), when reconstructed in a way that exposes its equivocal character:
1 If I know that the French Revolution really occurred in 1789 (in the adherent sense of reality), then I know that the world was (adherently) real before the year 2000.
2 I do not know whether the world was (adherently) real before the year 2000.
3 Therefore: I do not know whether the French Revolution really occurred in 1789 (in the inherent sense of reality). (1, 2 MT)
To ensure the credibility of the argument, two true premises dealing with adherent senses of reality were chosen. But we cannot really apply the modus tollens to them in order to reach the conclusion, because the sense of our attribution of reality in the conclusion is different from the sense of this attribution in the premises, making the conclusion equivocal and consequently fallacious. The argument would also remain equivocal in the case where we understand the senses of reality in the first premise as inherent. And although the argument would be sound and unequivocal if all the attributions of reality were understood as adherent, the conclusion would be trivial, for we do not need any argument to be aware that we cannot know, in an absolute way, that the French Revolution occurred in 1789.
Why the principle of closure seems threatened
Sometimes the problem we have dealt with is simplified to arrive at three statements composing an inconsistent set:
(2) K(p → ~h),
This is paradoxical, because although each of the three statements seems true, one of them must always be false. So, the skeptic denies (1), since he accepts ~K~h, which with K(p → ~h) entails ~Kp. The anti-skeptic denies (3), since he accepts Kp, which with K(p → ~h) entails K~h. There are also more neutral philosophers who reject link (2) in order to accept (1) and (3). These philosophers do this by rejecting the principle of closure under known entailment, which states that ‘Kp & K(p → q) ├ Kq’. Since the principle of closure is intuitive, and since K(p → q) seems to be the same as (Kp → Kq), so that the principle turns out to be ‘Kp & (Kp →Kq) ├ Kq’, which is obviously true, the rejection of this principle is a high price to pay.
By subjecting this paradox to our analysis of the kinds of attributions of reality, we can conclude that the three statements can be true without impairing the principle of closure. The argument is the following: Based on our analysis, the first statement, Kp, is true only when understood as attributing inherent reality to what p asserts. The third statement, ~K~h, is true when understood as concerning a lack of knowledge of the ultimate adherent reality of the world considered by h. The second statement, K(p → ~h), could be true if the reality of the antecedent and the consequent were both seen as adherent. This is shown by the following instantiation:
(1’) I know that I have two inherently real hands.
(2’) If I know that if I have two adherently real hands, then I
am not an adherently real brain in an adherently real vat.
(3’) I don’t know whether I am not an adherently real brain
in an adherently real vat.
From (2’) and (3’) we can conclude by modus tollens (4’), ‘I don’t know whether I have two adherently real hands’. Nonetheless, since this does not result in the denial of (1’), it is clear that the set is not just composed of true statements, but is also a consistent one.
If my argument is in its essence correct, then the contemporary forms of the skeptical and anti-skeptical arguments about our knowledge of the external world only seem to make sense, not because of a lack of logical syntax, but because this syntax is frivolously used, without enough semantic and pragmatic reflection.
 See Peter Unger, Ignorance: A Case for Skepticism (Oxford: Oxford University Press 1975), chap. 1.
 If you are impressed by Hilary Putnam’s argument against the possibility that you are a brain in a vat you can consider a different skeptical hypothesis or suppose that you are a recently envated brain. Nonetheless, we need to remember that Putnam’s argument is controversial. He argues that a brain in a vat cannot have thoughts about brains, vats, water…, because it has no causal contact with these things or their components. To strengthen this idea he imagines a brain in a vat that was generated by mere cosmic chance, so that there is no programmer who had contact with brains, vats, water, so that there could be a causal source… In this case, he thinks, the reference to water would be as illusory as the word ‘Churchill’ causally written by an ant walking on sand… Since we have thoughts about brains, vats and water, we cannot be brains in vats.
It is sometimes maintained that the problem with Putnam’s argument is that it ignores the plasticity of language. The argument unreasonably assumes that the representation causally generated in a brain as a copy of some electrical pattern in a computer cannot have a referential function analogous to the representation generated in a brain by the experience of the real thing, since only the latter representation refers to the real thing. However, why cannot the first representation be referential too, although in fact misleadingly referring to something existent only as an electrical pattern in the computer? We can compare Putnam’s reasoning with the case of a copy that we make of an old picture, believing that the old picture was a portrait of a real person. Suppose now that we discover that the old picture was a product of an artist’s imagination or that it was generated by pure cosmic chance… Of course, our copy could still be called referential, although only regarding the old picture. According to Putnam’s way of thinking, however, it could not be a copy of anything, since there was no original model. But this would be misleading. Moreover, the comparison between the events in the brain in a vat and the case of the accidental writing of the word ‘Churchill’ by an ant is also misleading, since the ant isn’t being causally led to copy anything. However, the brain in a vat is already causally led to copy the electrical patterns implemented (or simply occurring) in the computer. In this context, the necessary opposition between the subjective and the objective is given between the representations in the brain and the electrical patterns in the computer. Furthermore, even here there is something objectively responsible for the opposition between the subjective and the objective, namely, the brain and the computer. See Hilary Putnam, Reason, Truth and History (Cambridge: Cambridge University Press 1981), chap. 1.
 Immanuel Kant, Kritik der reinen Vernunft, B 106.
 Rudolph Carnap, ‘Empiricism, Semantics and Ontology’, in his Meaning and Necessity: A Study in Semantics and Modal Logic (Chicago: University of Chicago Press 1958), p. 207.
 R. Carnap, ‘Empiricism, Semantics and Ontology’, ibid., p. 214.
 Barry Stroud, The Significance of Philosophical Scepticism (Oxford: Oxford University Press 1984), chap. 5.
 See P. F. Strawson’s comments on Stroud in his ‘Skepticism, Naturalism and Transcendental Arguments’, in his Skepticism and Naturalism: Some Varieties (New York: Columbia University Press 1985), p. 7.
 T. S. Eliot, ‘Unreal city under the brown fog of a winter noon’, The Waste Land, ‘The Fire Sermon’ 200-210.
 Wittgenstein’s thesis was stronger, since in many cases he identified meaning with usage. See his Philosophische Untersuchungen (Frankfurt: Suhrkamp 1983), sec. 43. More precisely, he has identified meaning with the ways of use (Gebrauchsweise) or ways of application (Verwendungsweise) of words, and not with their episodic uses. See Über Gewibheit (Frankfurt: Suhrkamp 1983), sec. 61. This allows us to identify these ways (Weise) with semantic rules (or combinations of rules) determining the episodic uses. Examples of such rules would be the so-called criterial rules, which will be further considered in this text.
 This is why Wittgenstein has much to say about skepticism, although I do not agree entirely with his views about it, and I do not adopt them here.
 For Wittgenstein, criteria ‘give our words their common meaning’ (The Blue Book, Oxford: Basil Blackwell 1958, p. 57). His doctrine about criteria is scattered throughout his manuscripts. Important passages are found in The Blue Book, pp. 24-25, in his Philosophische Untersuchungen, sec. 354, in Zettel (Frankfurt: Suhrkamp 1984), sec. 438 in Wittgenstein’s Lectures –
1932-35 (New York: Prometheus 1979), p.
28. It is worth noting that the thesis that criteria are constitutive of
meaning would have no point if
we only had in mind objectively given criteria. However, by criteria he also
means the conditions intrinsically demanded in the criterial rules (or combinations of rules, which is a way we can
understand his doctrine of meaning as a calculus) – which should be conceptual
and verificational semantic rules – and not the objectively given criteria that
might or might not satisfy these rules, making them applicable or not. For an
investigation of criterial rules and their semantic role, see G.P. Baker,
‘Criteria: A New Foundation for Semantics’, in Ludwig Wittgenstein: Critical
Assessments, ed. Stuart
Shanker (London: Croom Helm 1986), vol. 2, pp. 194-225. See also the last
chapter of P.M.S. Hacker, Insight and Illusion: Themes in the Philosophy of
Wittgenstein (Oxford: Thoemmes Press, 2nd. ed. 1986). Cambridge
 John Locke, Essay Concerning Human Understanding (ed.) P.H. Nidditch (Oxford: Oxford University Press 1975), Book IV, chap. 11.
 George Berkeley, Three Dialogues Between Hylas and Philonous (1713), Complete Works ed. by A. A. Luce & T. E. Jessop (London: Thomas Nelson and Sons 1948-57), III, p. 235.
 David Hume, A Treatise of Human Nature, Book I, Section 1.
 Immanuel Kant, Prolegomena § 16.
 J. S. Mill, An Examination of Sir William Hamilton’s Philosophy (London: Longmans, Green & Co. 1889) chap. XI.
 Gottlob Frege, ‘Der Gedanke: eine logische Untersuchung’, originally published in Beiträge zur Philosophie des deutschen Idealismus, 2, 1918-19, pp. 58-77.
 G. E. Moore, ‘The Meaning of Real’, in his Some Main Problems of Philosophy (London: George Allen & Unwin 1953).
 Sigmund Freud, ‘Formulations on the two Principles of Mental Functioning’, in The Standard Edition, ed. by James Strachey (London: Hogarth Press 1958), Vol. 12.
 Thus, Laurence BonJour objects to the criteria of reality proposed by Locke. See BonJour’s book, Epistemology: Classic Problems and Contemporary Responses (Lanham: Rowman & Littlefield 2002), pp. 130-135.
 Of course, if they are not given and we think that they are given, they do not warrant anything. The concept of criteria in a definitional sense (primary criteria) is explained by Wittgenstein in The Blue Book, p. 24.
 As he writes in his Metaphysics: ‘There are many senses in which a thing may be said to “be,” but they are related to one central point, one definite kind of thing, and are not homonymous. Everything which is said to be “healthy” is related to health, one thing in the sense that it preserves health, another in the sense that it produces it, another in the sense that it is a symptom of health, another because it is capable of it’. Met.1003a 33-37.
 A precise and detailed reconstruction of the ways in which we obtain knowledge of external reality would require an empirical study of how the concept of external reality is learned, etc. Since my purpose here is to make a general defense of philosophical realism and to answer skeptics, my suggested reconstruction must suffice.
 Irwin Copi, Introduction to Logic (New York: Collier-Macmillan 1972), p. 93.
 Barry Stroud, The Significance of
Philosophical Skepticism, ibid., chap. 5.
 More plausibly, David Deutsch has argued for the greater simplicity of the hypothesis that our world is the ultimately real one. According to him, skeptical hypotheses require a world that is actually far more complicated than the idea that our world is the real one, for they are parasitic to the first one. This would require more complexity than that found in the external world as we know it. Consequently, the skeptical hypothesis is not as simple and economical as what he calls the commonsense view and for this reason should be rejected. (David Deutsch, The Fabric of Reality, New York: Penguin Books 1995.) However, simplicity is an epistemic virtue insofar as it is theoretical, as when we make comparisons between scientific theories with a similar scope. Simplicity does not seem to be an epistemic virtue when it concerns is factual simplicity of concrete states of affairs unknown to us. Compare the claim that there is only one egg in a basket with the claim that there are twelve eggs. Just because the first alternative is factually simpler does not make it more probable than the second. Similarly, since the simplicity considered by Deutsch is factual rather than theoretical, it does not seem to count as a reason for the truth of the statement that the reality of our external world is the ultimate one.
 This sentence sounds sufficiently intuitive to me. However, I believe that it should be analyzed as the following conditional: In a system where I could know that I had two adherently real hands, I would probably also know that I was not, in its adherent reality, a brain in a vat, but an adherently real human being possessing an adherently real body, etc.
 In fact, religions have contrasted our feeble world of sin with the real one, which is beyond our senses, and much of the philosophical tradition, from Plato to Plotinus and others, has tended to treat our world as the less real one. There are social-psychological reasons for this attitude, also examined by philosophers from Nietzsche to Herbert Marcuse.
 It would be hurried to conflate my proposal with present forms of contextualism. I do not base my considerations on the different strengths of knowledge (see Keith DeRose, ‘Solving the Skeptical Problem’, Philosophical Review 104, 1995, pp. 1-52), nor do I maintain that the context changes the ‘angle’ of scrutiny (Michael Williams, Unnatural Doubts: Epistemological Realism and the Basis of Skepticism, Princeton: Princeton University Press 1996). What I am doing is changing the focus from the concept of knowledge to the concept of external reality, investigating its (contextually relative) uses or senses in terms of criteria of application. In this way, my approach is not incompatible, I believe, with moderate foundationalism.
 G. E. Moore, ‘Proof of the External World’, in G. E. Moore, Selected Writings, ed. Thomas Baldwin (London: Routledge 1993 (1939)), pp. 165-6.
 See Bertrand Russell, Analysis of Mind (London: Routledge 1989), lecture 9.
 Certainly, we can construct an anti-skeptical modus ponens counterpart of this argument too, which is correspondingly equivocal.
 The validity of the principle of epistemic closure has been challenged by anti-skeptical arguments, particularly those of Robert Nozick (Philosophical Explanations, Oxford: Oxford University Press 1981), pp. 240-5 and Fred Dretske (‘Epistemic Operators’, Journal of Philosophy 67, 1970, pp. 1007-1023). My treatment of the skeptical problem leaves this principle untouched.