Freedom and Bondage in Psychology

 

 

Chomsky has long urged that the use of language is stimulus-free. The point in itself is obvious, as many of the most important points are, but it stands opposed to entrenched ideas. I will make some remarks about its interpretation and significance. The notion of a stimulus was introduced in the nineteenth century in connection with psychophysics. Just as a physical cause leads to physical effects that can be studied and measured, so a physical cause leads to psychological effects that can be studied and measured. Just as we can establish quantitative physical laws relating physical magnitudes, so we can establish psychophysical laws that relate physical and psychological magnitudes. For example, we can investigate how visual sensations depend on physical properties of external objects, discovering that the intensity of the former varies with the intensity of the latter (The Weber-Fechner law). Here the stimulus is light and the response is a visual sensation. We thus arrive at the idea that physical stimuli produce and are lawfully correlated with sensations conceived as internal psychological occurrences: we can predict the latter from the former according to quantitative laws. The stimulus elicits the response—makes it happen in a predictable manner. This makes the relationship similar to that between purely physical causes and effects—as in Newton’s laws of motion. Just as mass and force lead to acceleration, so light intensity leads to visual vividness. We can thus model psychophysics on physics: both involve law-like relations between magnitudes, with stimulus and response acting like physical cause and effect. This gives the experimental psychologist something tangible to work on, namely mapping the laws that connect physical stimuli with psychological responses. A science can thereby be constructed.

            This basic model was extended by subsequent behavioral psychology: now the response is taken to be overt behavior not subjective quality, and the laws relate physical stimuli to observable behavior. But the basic theoretical model is the same: stimuli eliciting responses in a lawful quantitative manner. We might say that psychology, so conceived, is the science of the laws of elicitation—how stimuli elicit responses, reliably and measurably, just as physical causes elicit physical effects. Thus stimulus-response (S-R) psychology was born: the experimenter could vary the stimuli at will—their intensity, frequency, probability, and timing—and discover the properties of the response. S-R psychology is a kind of physics of the behaving subject. From this perspective it is not that the response is behavioral that is crucial (it wasn’t for psychophysics); it is that the response is something elicited by the stimulus—caused by it, predictable from it, tied to it. Hence theorists took to speaking of evokingresponses by stimuli or stimuli triggering responses: responses are stimulus-dependent, stimulus-bound, stimulus-controlled. There is no response but that a stimulus makes it so—nothing is free from stimulus bondage. It might take some ingenuity to find the stimulus, and we may have to accept merely stochastic dependencies, but fundamentally all behavior is determined by outside stimuli. The brain (mind) is accordingly an S-R mechanism—a device for transmitting stimuli to responses; it plays no autonomous role in generating behavior—everything traces to those eliciting stimuli. Everything psychological works in the same way as it does in psychophysics—this is the model and paradigm.

            It is against this background that Chomsky’s point is to be understood. For his point is that language use—speech acts, utterances—do not conform to this model, contrary to the precepts of S-R psychology. What a person says on an occasion is not controlled by the stimuli impinging on that person—not elicited, evoked, or triggered. It may be semantically related to the environment of the speaker, but it isn’t fixed by that environment—the person might say something completely different in an identical environment. Still less is the utterance shaped by the surrounding stimuli: it doesn’t vary according to variations in the stimulus, say by growing louder or softer, or more or less interesting. The properties of the “response” have nothing to do with the properties of the “stimulus”. We should drop talk of stimulus and response here altogether: the utterance is not a response to anything in the array of impinging energies. The S-R model is quite inappropriate to the case. It follows that the mind-brain is not an S-R mechanism, at least where language is concerned. It works according to quite different principles. Nor is this a matter of merely statistical S-R relations: there is no law at all connecting environment to utterance. The environment might suggest a remark to the speaker, but there is no sense in which it elicits a remark. By contrast, a physical stimulus can elicit a perceptual response, but it doesn’t “suggest” such a response. On no account should we conflate these two relations. Linguistic behavior therefore does not fit the S-R model, but requires a quite different theoretical treatment.

            All this strikes me as unexceptionable and I won’t defend it further. I am interested in extending the point beyond language and in exploring the resulting picture of the human mind (and maybe the minds of other animals). First, we should note that the point is not that all behavior or psychological response is stimulus-free: much of it is stimulus-bound and fits the S-R model. The patellar (knee) reflex is an obvious example, along with the blink reflex and the salivation reflex. Chomsky’s point is that speaking is not like that. Conditioned reflexes are the same: stimulus-bound not stimulus-free. Likewise for perceptual responses: the perception is elicited by the stimulus (distal or proximal) and its properties depend on those of the stimulus (though not exclusively). The perceptual systems are S-R systems (which is not to say they are simple or mechanical). What you see is what you are made to see not what you make to see; but you make utterances and are not made to. The mind is therefore composed of two sorts of system: bound and free. It is not all bound or all free, but a combination. Just as speaking is not like seeing, so seeing is not like speaking: seeing an object is not like speaking in its presence—it is not a kind of commentary on the passing show. It doesn’t have that kind of freedom; it is not a kind of language (“That looks a duck”). We must not make an error of assimilation in either direction. The mind is both reflexive and reflective; both bound and unbound.

            What else is stimulus-free? I suggest that thought is. What a person thinks is not elicited by his or her environment, though it may be suggested by it. Thoughts are not responses to environmental stimuli. A person can think indefinitely many different thoughts in the same stimulus situation, since thoughts do not occur because of triggering by the stimulus situation. Their causation (if that word is appropriate) is endogenous. Your thoughts can roam far and wide while your senses are stimulated thus and so. There is no supervenience of thought on outer stimuli. A stimulus may prompt a particular thought, but it may not; thought operates autonomously, without reliance on triggering stimuli. Beliefs may be elicited by perceptual stimuli, but thinking is another matter. So thought is like speech: stimulus-free and endogenously generated. Speech is an external action while thought is something internal, but both differ from perception and reflexes. In fact, I am inclined to suggest that language is stimulus-free because thought is stimulus-free: our thoughts lead to our speech acts (though not as stimulus to response) and our thoughts are stimulus-free, so our speech acts are likewise stimulus free. We say what we do because of what we think, but what we think is not controlled by impinging stimuli (unlike what we perceive); therefore free speech (in this sense) is underwritten by free thought. At some point in evolution the mind detached itself from S-R relations (a happy mutation!) and developed operational autonomy, thus producing thought; language then built upon that foundation, developing its own autonomy. The decoupling of world and mind is what stimulus freedom amounts to, and both thought and language share it. If we conceive of thought in terms of the language of thought, then the autonomy of thought rests upon the autonomy of the internal language—utterances in the language of thought are stimulus-free. But then it is the stimulus freedom of the inner mental utterance that is basic, not the outer vocal performance. In any case, language and thought are clearly intertwined when it comes to the impotence of the stimulus. And this is not a matter of the “poverty of the stimulus”, because thought and speech are not responses to stimuli at all, however poor and meager. They don’t result from an inadequate stimulus but from no stimulus. S-R psychology simply doesn’t apply to them.

            A certain kind of dualism takes shape around these observations—a dualism within the mind. Human minds (and maybe others) have two very different kinds of faculty, according as the faculty is stimulus-bound or stimulus-free. S-R psychology applies to one sort of faculty, including perception and motor reflexes (conditioned and unconditioned), but it fails to apply to another kind.  [1] We must not try to assimilate one kind to the other in either direction. We would therefore expect that different sorts of theory are appropriate to the two sorts of faculty, with one sort mirroring theories in physics and the other not. As I remarked, psychophysics modeled itself on Newtonian mechanics; but the theory of thought and speech will not be like that, because of inherent stimulus freedom. This prompts the question of why such a division exists: what is it about speech and thought that makes them stimulus-free? Is there something about their intrinsic structure that leads to their stimulus freedom? The obvious place to look is productivity—the potential infinity of sentences and thoughts grounded in finitely many combinable elements. Is this essential creativity the reason for stimulus freedom? Or are these two independent features of speech and thought? The natural hypothesis is that they could not be productive in the way they are if they were stimulus-bound. Suppose we try to imagine a speaking creature whose speech acts are as rigidly governed by eliciting stimuli as our reflexes and perceptions are. This creature cannot speak unless it is triggered into doing so by an impinging stimulus, as we cannot see without being stimulated to see. Its verbal behavior is subject to S-R psychology. And this is not just a motor limitation; it can’t even enunciate sentences in its head without an external stimulus. This is hard to get one’s mind around, so alien is it to our own relation to language. One feels the creature would just be blurting noises out, not speaking. We construct our sentences as we utter them (or a bit before), but these putative speakers are not constructing their utterances, just emitting them.  [2] We actively produce our utterances according to plan, but they passively emit noises when a stimulus strikes them. They can’t help speaking, but they can’t speak at will either. How could their utterances be productive if they don’t produce them—if they are extracted not created? The faculty of speech is essentially creative, given productivity, but then it can’t be elicited by stimuli in the way perception and reflexes are. Thus speech is necessarily stimulus-free. Active combination is its essence, but that is not compatible with the S-R model. We make utterances; they are not elicited from outside. And the same is true of thought for essentially the same reasons: thoughts are actively constructed complex entities not pre-existing fixed entities; so we can’t conceive of them as triggered in the S-R style. If thoughts were so triggered, they would not be thoughts, but something more like perceptions. What it is to be a thought involves stimulus freedom; this is not a contingent feature of thoughts. Thought, like speech, is essentially free—not an inescapable response to a exigent stimulus.  [3]

            We can therefore say that the form of sentences and thoughts is internally related to their being stimulus-free. If we call this form “grammar”, we can say that grammatical events are necessarily stimulus-free: what is composed in a certain way must come about in a certain way—freely, not by bondage to a stimulus. Form dictates etiology. Given that sentences have grammar, they must be stimulus-free; the price of stimulus bondage is loss of grammar. We can certainly imagine a machine that produces sounds like the sentences of a human language when stimulated to do so, but that is not to say that those sounds have grammar. In order to have grammar the sounds must be produced in a certain way (a derivational history), and that way excludes stimulus elicitation. The question is like asking whether genuinely creative acts could occur as responses to stimuli, and the answer is that these are incompatible attributes. Likewise, it is strange to suppose that a perceptual state could come about without a triggering stimulus—in the way that we speak and think. For that is incompatible with its defining attribute: a free-floating visual percept, produced at will, independent of any eliciting stimulus, would not be a genuine percept, but something more like a markedly visual thought. Perceptions must indicate the actual impinging environment and hence be responses to stimuli from that environment; they cannot be stimulus-free in the manner of language and thought. You change the nature of the thing if you change its mode of occurrence (or the laws of its occurrence). Language and thought, with their distinctive intrinsic structure, don’t just happen to be stimulus-free–as perception doesn’t just happen to be stimulus-bound. This distinction of mode of occurrence marks a deep ontological division in the mind. We might even say that we have two minds: an S-R mind and a non-S-R mind. We feel this division in ourselves all the time, as we register perceptions triggered by the environment and entertain thoughts stemming from who knows where. We consist of reflexes and spontaneities—bondage and freedom. We are slaves to stimuli and yet we are able to rise above them; we are not purely one thing or the other.

 

  [1] Readers who sense an affinity between what I say here and Fodor’s The Modularity of Mind are not mistaken. Encapsulation and reflexivity are connected properties.

  [2] Reflexive vocalization occurs in us too: we blurt out a vocal response to a stimulus, e.g. when injured and in pain. But these elicited vocalizations are not structured utterances, merely cries. This is not reflexively triggered speech.

  [3] I have not discussed other mental phenomena that raise the same question: dreaming, mental imagery, seeing-as, memory, knowledge of various kinds (perceptual, mathematical, moral, modal, introspective), logical reasoning, desire, emotion, intention, aesthetic response, and whatnot. About each of these we can ask whether they are stimulus-free or stimulus-bound (or possibly a mixture of the two), but I won’t attempt to answer this list of questions now–except to state my view that dreaming is free and memory is bound.

                                   

 

 

 

 

Consciousness and the Eye

 

 

If you look into the eye of an animal (say, a bird) you gain a strong impression of consciousness: the eye seems to brim with consciousness. The same is not true of other sense organs: you don’t get a comparable impression from looking at the ears or nose or skin. This suggests that there is particularly close connection between vision and consciousness—vision is par excellence an exemplar of consciousness. The eye is a receptor that responds to light, a transducer that converts an impinging light stimulus into nervous activity, and ultimately into a conscious impression of the world. Thus there is a close connection between light and consciousness. Elsewhere I have suggested that light is the reason that consciousness exists on planet Earth: because of light from the Sun evolution designed an organ that could respond consciously to this rich source of information, and no other stimulus could trigger such a radical biological innovation.  [1] I call this the “optical theory of consciousness”. The optical theory says, crudely, that light causes consciousness; less crudely, that light-sensitive tissue is the kind that uniquely gives rise to consciousness. No light, no consciousness; with light, consciousness predictably develops. I will make some further remarks in defense of this theory, particularly with respect to the uniqueness claim.

            Light has some claim to be the most remarkable physical phenomenon in the universe (in some sense of “remarkable”). The eye has some claim to be the most remarkable biological adaptation, and consciousness the most remarkable psychological phenomenon. Clearly, there must be a reason that the properties of light lead so reliably to the existence of eyes (the convergent evolution of eyes is well known): there is something about light that makes it worth responding to—and worth being conscious of. It is a stimulus that craves a response—that demands to be recognized. You would be a fool to take no notice of light, biologically speaking. Hence animals that live in light tend to be sensitive to it; and there are eyes everywhere, beady and acute. But what exactly is so special about light? We know its speed is special, being both constant and maximal. That sets it apart from everything else—the character of its motion. But it is hard to see how this property of light would be biologically significant: even if light were slower or its velocity variable, it would still be the stimulus that it is from the organism’s point of view. Light also travels in a vacuum, divides into a spectrum of wavelengths, has particle-wave duality, and has zero mass—but again, these properties have little to do with light as a biological stimulus. The feature of light that is most relevant to its stimulus profile is surely its informational density: there is just so much information present in the light that impinges on the typical retina. This is because of its physical structure—all the billions of photons reflected off surfaces bearing fine-grained information about those surfaces. This is a powerful resource for organisms to tap into—a detailed map of what is going on around the organism. The claim of the optical theory is that consciousness arises when a threshold of informational complexity is reached, but not before, and that light alone reaches that threshold. I won’t repeat the arguments for this view now, but I do want to address some potential objections that cry out for answers.

            The most obvious objection is that vision is not the only conscious sense. How then could light uniquelycause consciousness to evolve? Don’t sound and physical contact also cause hearing and touch to evolve? Aren’t these also associated with states of consciousness? The optical theory must reply that consciousness originallyarose in connection with eyes and only subsequently spread to the other senses. That may sound ad hoc, but actually it is not so far-fetched. It is quite commonplace for an adaptation to arise in a certain way and then become repurposed for other functions—as when feathers arose for thermal regulation and then were co-opted for flight. Maybe consciousness came with eyes alone but then the other senses recruited it as an enrichment—before that they operated without consciousness. Given that there can be subconscious perception, this is perfectly possible. Similarly, other properties common to the senses might have arisen originally in connection with vision, but were then distributed more broadly: that might be true of perceptual segmentation or perceptual computation or perceptual constancy. Segmentation is useful in visual perception and it may have started life in that domain—indeed, it may not have been possible in a different domain as an initial adaptation. We have trait transfer following trait origination. Evidence for this would be telltale remnants of the origin of the trait—say, visual-auditory synesthesia. Maybe such synesthesia was more common soon after hearing borrowed consciousness from vision, later phased out as pointless. Or there might be abstract structural features of visual consciousness that show up in auditory consciousness, bearing witness to their origins (“quality spaces”). Or there may be metaphors drawn from vision and applied to the other senses. So the objection is not as decisive as it may sound—the optical theory has potential resources with which to explain the spread of consciousness to non-visual senses. If we grant that the non-visual senses don’t command stimuli that inherently cross the threshold of informational density required to create consciousness, then the optical theory might be the only way to explain why they are nevertheless accompanied by consciousness.

            A sharper test of the optical theory is supplied by the physiology of actual animals on Earth. The theory would be decisively refuted by the existence of two sorts of animal: those with eyes but no consciousness, and those with consciousness but no eyes. Such animals would show that eyes are neither sufficient nor necessary for consciousness—seeing automata and blind sentience. A survey of the animal kingdom fails to turn up any animals of the former kind: no animals both have eyes and are clearly not conscious. You might point to the compound eyes of insects, but (a) it is not clear that insects are devoid of consciousness and (b) we could always restrict the optical theory to more sophisticated eyes (such as the camera-like variety). It is a striking fact how well correlated eyes and sentience are—we don’t find animals with developed eyes going around like zombies! In fact, the animals with the best eyes always seem particularly alert and conscious—birds, for example. In contrast, the dull and sluggish eye tends to be correlated with manifest somnolence, a drowsy mode of being. But the second kind of animal is more challenging for the theory: what about animals that see nothing but feel at lot? We talk about being “as blind as a bat”, but we don’t hesitate to call bats conscious. Why can’t there be an animal with acutely conscious hearing but no seeing eyes? Isn’t that the case for the human blind?

            The answer to this worry is that the optical theory does not claim (absurdly) that consciousness is only possible in the sighted; it claims that consciousness arises in evolution specifically from the visual sense. So it is entirely possible for sight to be reduced or eliminated in a species as a result of changing environmental conditions, despite the fact that it arose in a visual setting. A genuine counterexample would have to be an animal with no sighted ancestors that is nevertheless fully conscious. Bats have vision and may well have evolved from animals with better vision (not being nocturnal), so their consciousness could easily have arisen from vision, and only from vision. Similarly for blind snakes and moles: their evolutionary history might well have included sighted ancestors. I don’t know of any animals that are clearly conscious but have nothing visual in their present make-up or evolutionary history; so vision could be the sole source of the brain structures that make for consciousness. Those brain structures may persist in a biological line while eyes themselves are phased out. There are certainly organisms that have blindness written deep in their genes—such as bacteria, viruses, plants, plankton, jellyfish, and eyeless worms—but they are all strong candidates for zombie status. The correlation between consciousness and eyes is strong, and not coincidental according to the optical theory. It is also strong between other sense organs and consciousness, but the optical theory claims that consciousness arises (originally) only in the context of stimuli that reach a certain threshold of informational density, which light alone reaches.

The kind of density found in a typical visual experience is never matched by the density found in an auditory experience—simply because packets of light are physically much richer than sound waves. The stimulus causing your eardrum to vibrate has nothing like the complexity of the stimulus falling on your retina, and similarly for tactile and olfactory stimuli. The brain will process a stimulus unconsciously if it can (consciousness being biologically costly); it will only invoke consciousness if the stimulus is so complex as not to be susceptible to unconscious analysis. Natural selection has apparently decided that the costs of consciousness are worth it in order to extract encyclopedias of information from the visual stimulus. If the planet were perpetually dark, there would be no need for consciousness: eyes would be pointless and the other sorts of physical stimulus don’t call for anything beyond unconscious processing. Imagine that those stimuli were even more impoverished than they are now, with precious little information contained in them: the atmosphere might be thin and unresponsive, so that sound waves possess little structure or energy; or the chemicals emitted by objects might reach the nostrils in small numbers. A relatively simple sensory mechanism will suffice for maximum information extraction from these limited stimuli; there is no need for the elaborate and metabolically expensive machinery of sensory consciousness. Well, compared to light stimuli, actual-world non-visual stimuli are similarly impoverished—as a matter of the simple physics of the stimuli.

Let me drive the point home by distinguishing synchronic and diachronic informational density. We have not only the photons currently striking the retina; we also have the photons striking it a moment later, updating the perceiver’s image of the surrounding world. This happens with inconceivable rapidity as battalions of photons continuously bombard the retina; the information conveyed concerning changes in the environment is virtually incalculable (just think of catching a ball). This is a colossal task of perceptual analysis, orders of magnitude beyond what can be achieved with the ears (not that this is unimpressive). Echolocation is remarkable, but it is not to be compared to vision, which is presumably why so few animals substitute the one for the other. The auditory stimulus is simply not rich enough, no matter how adept the animal may be at echo detection. Sounds are mediated by perturbations of molecules in the atmosphere, a fairly crude way to transfer information, but light has limitless bandwidth and extreme fineness of structure. The retina has to absorb what this stimulus imparts, then have it instantly replaced by a new stimulus, keeping track of the changes. The optical theory says that it is this challenge that brings sensory consciousness into existence. We don’t know how this happens, or even why it needs to happen, but evidently that is the way things work. It just seems to be a natural fact about light and consciousness that the former triggers the latter (given the right evolutionary context). If we subscribe to panpsychism, we could say that the consciousness-creating properties of matter would remain latent if it were not for vision.  [2] The other senses don’t need to go beyond the non-conscious aspects of matter to perform their work, but vision needs more advanced machinery to perform its work (why, we don’t know). If the world were completely dark, the psychic properties would not coalesce into conscious experience, but light operates to elicit them to produce such experience. However consciousness may be generated, the optical theory says that light-sensitivity is the original trigger that causes it to emerge. We know that light provides the energy that drives the entire biological world (via photosynthesis) but it also turns out that it drives the emergence of consciousness in the psychological world (what we might call “photo-sentience”).

If the optical theory is on the right lines, a good place to look to gain understanding of consciousness would be the psychophysics of light. This has been extensively studied, particularly with respect to receptor cells in the retina (rods and cones etc). But a more self-conscious study of light as it relates to consciousness might prove helpful: particularly the kind of internal structure both have. Is it a granular or a continuous structure, or possibly a mixture of both? Light seems to have both sorts of structure depending on the context (behaving sometimes like a wave, sometimes like a particle), and there is some reason to find the same sort of duality in visual consciousness. If consciousness evolved as an adaptive response to light, it might inherit some of the features of light, structurally if not in substance (consciousness is surely not made of light). Response mirrors stimulus; effect reflects cause. At least this looks like an interesting research program. The nature of the biological world is shaped by the fact that photosynthesis lies at the root of everything organic, so it is reasonable to expect that the psychological world would reflect the centrality of light in shaping the mind. Light is the aspect of the environment that has exercised the most profound influence on the nature of sensory experience (according to the optical theory). Consciousness is the progeny of light—what happens to organisms when light-sensitive receptors come into the world.

No doubt the optical theory is highly speculative, also counterintuitive in some respects, but it provides a novel way to think about a problem sorely in need of fresh avenues of inquiry. It also has a poetically attractive quality: light and consciousness turn out to be made for each other, in virtue of the brute physics of light. There is nothing in the universe quite like light, and there is nothing in the universe quite like consciousness: these two remarkable things turn out to be deeply connected.

 

Colin McGinn         

           

  [1] See “Consciousness and Light”, in Philosophical Provocations: 55 Essays (MIT Press, 2017). I will not here repeat the arguments for this position.

  [2] I put it this way for expository purposes not because I subscribe to panpsychism. I also hope it is clear that the optical theory is in no way incompatible with the idea that consciousness is mysterious, quite the contrary.

                                   

 

 

 

Matter and the Limits of Skepticism

 

 

The skeptic questions whether we know the external world exists, purporting to provide a proof that we don’t know that it does. The proof takes the form of describing a possible alternative to what we normally assume. Thus it is suggested that a deceiving demon is a possibility, or that we might be dreaming, or that we might be a brain in a vat. Since these are real logical possibilities that we can’t rule out, we must conclude that we don’t know that the world is as we normally take it to be. For instance, I don’t now know that I am sitting at a desk looking at a tree. It might all be one vast illusion or hallucination. It is important to this argument that the skeptical alternative be a real possibility, which must be accepted as such, or else we could simply reply that no such thing is possible, so there is nothing we need to rule out in order to know what we think we know. If the skeptic suggests something impossible, or not clearly possible, his argument is toothless; and the usual suggestions satisfy this condition, since they do describe real logical possibilities, often rooted in familiar facts (people can and do deceive us, we do dream, and we do suffer illusions and hallucinations).

            But what about our belief that matter exists? This is not the belief that the external world exists as we customarily think of it, but just the belief that matter of some sort exists. If I am a brain in a vat, then what I take to be the external world is all illusion, but matter still exists in the form of vats and brains. If I am in bed dreaming, then my material bed and body exist even if what I dream about doesn’t. The evil demon can be as material as you like and still deceive me. So the question of matter is very different from the question of the external world as normally conceived. Is it possible to be a skeptic about the existence of matter? Can the skeptic show that we don’t know that matter exists? Can he give a proof of this analogous to his proof that we don’t know the external world exists? Clearly he can’t do this by suggesting his usual logical possibilities, since these all presuppose the existence of matter, so they are not alternatives in which there is no matter. He needs a real possible world in which mind exists but there is no matter of any kind, in order to provide such a proof. He needs to provide a logical possibility that we can’t rule out.

            This is a much more difficult thing to do than the usual exercises in skepticism, because there is nothing familiar he can rely on—no actual cases in which we have mind without matter (as there are actual cases of sense impressions without external objects of perception). What has to be claimed is that it is possible to have disembodied minds in a wholly non-material world. And the problem is that this is not clearly possible and may well be quite impossible, so the skeptic has not discharged his obligation to produce a genuinely possible alternative to what we normally take for granted. We do not therefore need to listen to his argument; he has no argument against our belief that our minds exist in a world of matter. Nothing he has to say proves that we don’t know what we take ourselves to know. So this belief about the world beyond the mind is not vulnerable to skeptical challenge of the standard (distressingly convincing) kind.

            To clarify the point, let’s consider two other types of skepticism regarding the non-mental world, concerning time and space. Suppose the skeptic says that we don’t know that time exists: we have impressions of time, but there may be no objective time corresponding to these impressions. This, he contends, is a real possibility—that minds can exist without time existing. We would be within our rights to reply that this is not a real possibility: minds exist in time and must do so. For minds change and change requires time—our consciousness consists of temporally successive states. So the skeptic has not provided an indisputable logical possibility that is an alternative to the way we normally takes things to be; at best he has suggested a highly questionable metaphysical theory, namely that minds can exist without time existing. He has certainly not proved that this is a possibility. So he has not undermined our claim to know that time exists by pointing to a logically possible alternative in which we have the same impressions of time but there is no time.

Now consider space: the skeptic claims again that we don’t know that space exists, despite our impressions of space. The reason is that it is logically possible for minds to exist without space, and we can’t rule out the possibility that our minds are like that. But again, we can reply that it is not logically possible to have minds without space—minds necessarily exist in a world of space. Certainly the skeptic has not proved that his supposed alternative is logically possible.  [1] We might argue against him that minds cannot be individuated without space, since space provides the indispensable basis for establishing the numerical distinctness of minds—your mind is not identical to my mind because our minds are in different places. The skeptic might engage in a metaphysical argument with us about this, but he has certainly not suggested an indisputable possibility that we are obliged to accept. So we are not in the same epistemic predicament with respect to our belief in space that we are in with respect to our belief in the external world (as distinct from the world of matter). We are not vulnerable to skeptical doubt in the same way, viz. by exhibiting a clear logical possibility that our evidence fails to rule out.

            Similarly with respect to matter: if the skeptic says that there could be minds without matter, we can reply that we don’t agree with that, since minds require brains. These brains may not be as we normally conceive of them, but there has to be some material basis for minds—they cannot be purely immaterial. The skeptic may engage us in metaphysical argument at this point, but what he can’t do is point out that we ourselves accept his alleged possibility as a real possibility—as we do for the brain in a vat possibility. Whether we can prove that mind requires matter is not to the point; the question is whether he can prove that minds can exist without matter. For that is the possibility he needs if he is to show that we don’t know that matter exists. He needs a certain metaphysical possibility to undermine our confidence in what we ordinarily believe, but it is not clear that he has one—or rather, it is clear that he does not have one. So there is a significant asymmetry between this type of belief and the type of belief the usual skeptic questions. The usual skepticism does not then generalize to these further beliefs—in time, space, and matter. Our claim to know of the existence of these things has not been undermined, even granted that our claim to know the existence and nature of the external world has been undermined.  [2]

            It may be thought that this is a disappointing result, since we have made no inroads against the skeptic on his favorite territory: for all I have said, we still don’t know we are not dreaming, not a brain in a vat, and not the victim of a deceiving demon. But this is too pessimistic (or perhaps I should say too optimistic) because we have established a definite limit to the corrosive power of skepticism: there are beliefs about the non-mental world that are immune to skeptical challenge (at least of the usual kind). For all that the skeptic has said, we do have knowledge concerning the world beyond the mind—he has said nothing to undermine our confident belief in objective time, space, and matter. His skepticism is thus confined to a certain subset of beliefs about objective reality. I count this a strong anti-skeptical result, even if it falls short of what we might ideally hope for.

 

  [1] The skeptic can point out that we know of cases in which a sense impression has no corresponding object and then ask how we know this is not always the case, but he cannot likewise point to cases of minds without time, space, and matter and ask how we know our minds are not in this position. There are no such actual cases to point to. 

  [2] That is, our belief that we sense external objects and are not subject to massive illusion: this belief isundermined by the skeptic. But he has not undermined our more general belief in things outside the mind.