Mind-Dependence

Idealism is the thesis that the world is mind-dependent. In particular, the things we call material objects are dependent on the mind for their existence: to be is to be perceived. Realism is the thesis that the world is mind-independent, so that material objects can exist whether perceived or not. Here is a simple argument against idealism and in favor of realism: the mind depends on the brain, which is a material object, but no material object can depend for its existence on another material object; therefore, material objects cannot depend for their existence on minds. If material objects depended on minds for their existence, then they would depend on brains for their existence, because minds depend on brains; but brains are material objects, which would imply that other material objects depend for their existence on these material objects, but material objects never depend on other material objects for their existence; so material objects cannot be mind-dependent, and idealism is false. I think this is a valid argument, but it may be thought to beg the question: for can’t we say that brains also are mind-dependent? But suppose that were so: it would imply that the being of brains consists in their being perceived—but those very perceptions would themselves depend for their existence on a brain, and hence would call for a material object to exist that is not itself merely a perception. Perceptions of brains need brains too. Moreover, suppose that we had never perceived a brain and had no knowledge of the existence of brains: there would then be no perception of a brain for a brain’s existence to be dependent on. No, the existence of brains cannot be a matter of the existence of perceptions of brains; the existence of brains is an objective matter that is necessary in order for minds to exist. But since material objects can never depend for their existence on the existence of another material object, including a brain, they cannot depend for their existence on minds. The doctrine of “cerebralism” has zero credibility—that is, the doctrine that the entire material world depends on the existence of brains. No one supposes that tables and chairs, trees and mountains, depend on brains. That would not be idealism but a peculiar form of materialism—everything material is really one type of material thing. This is not the identification of reality with appearance, as idealism maintains, but the identification of reality in general with brain reality in particular (neurons, blood, brain chemicals).

Why don’t we see this point, thus rendering ourselves immune to idealism? The reason is that the dependence of mind on brain is opaque to us: we don’t have a grasp of how the mind depends on the brain, though we know that it does. Thus we think we can conceive of a universe in which minds exist but brains don’t, which contradicts the thesis of necessary brain dependence. But such intuitions are immediately suspect stemming as they do from ignorance of the mind-brain nexus. Not that the brain must be just as we conceive it, but there has to be something to do the job of the brain even if it isn’t exactly as we conceive this thing now (even Berkeley saw the necessity for finite and infinite spirits in addition to the ideas that exist in them). The fact is that (barring skepticism) we have discovered empirically that brains are the basis and sine qua non of minds, so that any dependence on the mind is also a dependence on the brain: but brains cannot be what material objects in general depend on, since material objects don’t depend for their existence on other material objects.[1]

Note that the idealist claim is never that the world depends on someone’s mind (unless that someone is God); it is the thesis that my mind is what the world depends on. The idealist never says that the world depends for its existence on (say) the mind of Justin Bieber: why him, we might wonder. But we are easily seduced into thinking that our own mind might be the one on which everything depends (the world is my world). Why? Because of a certain kind of egocentrism: we don’t tend to see ourselves as merely one subject among many. Once I see myself as something in the world along with other subjects, I see that the world as a whole cannot depend on me in particular—why me and not him? Putting this together with the point about brain-dependence, why should reality depend upon my brain, which is just one brain among many (many billions in fact)? Why should this particular material object (the one in this head) be the font of all being? My brain does not enjoy this momentous privilege, and neither does my mind. In the case of God we are more inclined to accept that God’s mind could be the basis of all being, but that is because we don’t tend to see him as merely one person among many, with no more power to generate reality than anyone else; also, we are far hazier about the kind of existence God possesses. But putting aside this kind of theistic idealism and sticking to the usual secular kind, there has to be a question about whose mind is doing the ontological work; and Colin McGinn’s mind is no more privileged in this respect than Justin Bieber’s mind (rather less so). I am just one subject among countless others, and my brain is one among a multitude of brains (cf. my kidneys), so reality as a whole can’t depend on my mind or my brain. I am something in the world; it is not in me. We only fail to see this because of a stubborn (and callow) egocentrism that insists on putting ourselves at the center of things.

Once we see that our minds depend on our brains, and that our brains are just material objects among other material objects, the appeal of idealism evaporates. The world can never reduce to my world, i.e. how it seems subjectively to me. There must be a reality external to my experience (starting with the brain). This would not need arguing if the dependence of mind on brain were written into our everyday lived experience, but that would imply that the mind-body problem has a readily accessible solution. In a way, the mind-body problem is at the root of idealism: a solution to the former would do away with the latter.[2]

Colin McGinn

[1] Spatially separate from them: of course, objects can depend for their existence on their parts. But it is not plausible to suppose that ordinary material objects have brains as parts. The general principle I am relying on is the standard notion of a material substance—that which does not depend on other substances for its existence (“self-subsistent”).

[2] I don’t wish to suggest that idealism is not a respectable philosophical position; indeed, I think that the debate between idealism and realism is one of philosophy’s deepest questions. I am simply pointing out its connection to the mind-body problem, also one of philosophy’s deepest questions. Idealism is as attractive as the mind-body problem is hard.

Perception and Emotion

Everyone knows that sensory qualities are associated with emotions, though the manner of association is obscure. Colors suggest emotions (red passion, blue sadness); sounds are perceived emotionally especially in speech and music; tactile sensations are felt as pleasant or unpleasant; smells can be appetizing or revolting; tastes delicious or nasty. It is a question what establishes these associations–whether they are innate or cultural, the result of arbitrary conditioning or of a perceived intrinsic connection. They seem to bypass explicit belief: it is not that we are of the opinion that screeching noises are irritating, or that rotten food tastes disgusting, or that green is soothing. Artists and musicians, not to mention chefs and masseurs, know how to exploit these emotional resonances. The sensory faculties are clearly hooked up somehow to the affective parts of the brain. We sense feelingly. The same must be true of other animals, even when cognition is not at the human level. Imagine how birds see colors or cats and dogs hear sounds (the bat may thrill to its sonar perceptions). These sensory qualities clearly have affective, motivational, and appetitive connotations for the animals that sense them. They are not emotionally neutral.

It is noteworthy that all the sensory qualities mentioned so far would be classified as secondary qualities, i.e. qualities originating in the mind (or brain) and projected onto the world. Let’s call these qualities subjective: then we can say that subjective qualities are apt to have an intimate connection to emotion. What we project we resonate to; what comes from us excites us in this way or that. But is the same true of primary qualities, particularly size and shape? Here we seldom hear talk of emotional associations, and scientific studies of possible such associations are few and far between. What is the emotional meaning of a straight line as opposed to a wavy one? Are circles evocative of different emotions from rectangles? Does size matter emotionally? Maybe some sort of affective meaning can be conjured up by remembered resemblance, but it doesn’t seem natural or intuitive (geometers might feel differently about circles and squares). So the following generalization suggests itself: objective qualities don’t have the emotional associations of subjective qualities. That is, the perceived qualities of things that are discovered not invented, received not projected, lack emotional resonance—or if they have it, it isn’t in the same way subjective qualities do. What doesn’t come from us doesn’t move us in the way what originates internally does.  Emotion-laden perception is confined to projected subjective qualities. Proponents of the distinction between primary and secondary qualities, past and present, have not noticed this difference, but it appears to exist—and it is not altogether surprising on reflection. For secondary qualities have been manufactured biologically as a tool of survival, so they are likely to have conative significance; but primary qualities exist in the world anyway, whether relevant to organisms or not. They might be relevant to survival, adventitiously so, but they are not guaranteed to be, unlike secondary qualities. The absolute conception is not emotionally imbued, save per accidens.

One can envisage an extreme reaction to this point, namely that secondary qualities are emotions, projected outward. To be red, say, is to be imbued with passion. That would be pretty crazy, to be sure, but it brings out an interesting point: perceptive and emotive theories are not necessarily opposed. In ethics people have discussed whether moral qualities are perceived or are merely reifications of our emotions: but they can be both. Maybe we perceive moral qualities and at the same time, inextricably, feel certain things (a rush of approbation, a surge of indignation). If perception is essentially emotional, it won’t be surprising to find that moral perception has a characteristic type of affect associated with it. And it is surely true that moral impressions (for want of a better term) are heavily emotion-laden, as well as motivating—which is what we would expect if they are perceptual in nature. It might even lend support to the idea that moral qualities are manufactured by the mind and projected outwards, not found among the objective features of the world, though perceived there. The indicated theory might thus be labeled “perceptive emotivism” or “emotive perceptivism”. This seems like a pleasant resolution of an old dispute.

It must be admitted that the connection between perception and emotion is far from pellucid. Why particular colors have they associations they do is puzzling and not at all self-evident; and it is not plausible that we are somehow taught to make these associations. Clearly that is not so for taste and smell. The mind is so configured that the senses and the emotions are coupled to each other in multiple and complex ways. Nerve fibers spanning cortical regions must be the basis for this, and these neural connections must have been established somehow. Why is the Blues called “the blues”? Why are we said to “see red”? Why are cowards described as “yellow” and novices as “greenhorns”? But some semblance of intelligibility arises from the observation that the sensory qualities with pronounced emotional meaning are mental projections—they come from the same place that emotions come from. Emotions are not objective qualities of external things—potential subject matter for physics—but neither are secondary qualities. The perceived world and the emotional world are inextricably intertwined, both having their origin in the subject. The objective world of primary qualities, by contrast, is emotionally neutral and not a mental projection. It has nothing intrinsically to do with the needs of perceiving organisms.[1]

[1] It is good for organisms to have accurate perceptions of primary qualities, but the qualities themselves are not reflections of the organism’s own nature—hence the lack of emotional punch. Subjective qualities, by contrast, are bound up with the inner life of the organism. It is as if they are emotions distilled and objectified.

Are Reasons Causes?

It used to be held in the 1950s that reasons are not causes (the “logical connection” argument), but the tide turned in the 1960s. The new orthodoxy was that rational explanation of actions is a species of causal explanation: beliefs and desires are the causes of action.[1] True, this is a special kind of causal explanation, since reasons have a rational structure exhibiting logical connections; but that is no bar to their also being causes (efficient causes, in Aristotle’s terminology). Beliefs and desires cause actions in the same sense that impacts cause motions or smoking causes cancer. Since a reason is a combination of a belief and a desire it too causes actions: it causes what it rationalizes. I go to the shops intending to buy fruit, desiring some fruit and believing that the shops are the place to go; these mental states cause my shop-going behavior. It is rational to go to the shops if you desire fruit and you believe the shops are the place to get it, and these attitudes operate causally to bring about the behavior in question. The attitudes are causal factors, triggering events, effect-producers; no doubt they are underlain by brain causes that exploit efferent nerves and send neural volleys. What we have here is just another example of multi-factor common-or-garden causation.

But another point about reasons and actions also gained traction at around this time, namely that desires and beliefs cannot act on their own—they must always work as a couple.[2] Suppose I leave the house with an umbrella: you might surmise that I do so because I believe it is about to rain. But that is only a rational action if I also desire to keep dry: I might have that belief and fancy a dousing, so I leave the umbrella at home. Similarly, a desire to stay dry will only lead me to carry an umbrella if I believe umbrellas are a good way to keep dry. The desire supplies an end, but it takes an instrumental belief to determine a particular type of action. The action is a means to satisfying a desire, but it takes a belief to link the desire to a suitable action. This is a kind of confined holism whereby beliefs and desires only cause actions by conjoining with each other. The point I want to emphasize is that the components of reasons (beliefs and desires) have no intrinsic causal power with respect to action. Logically they cannot bring about actions individually but must rely on each other to generate an action. It is not that they have no specific causal powers considered singly; rather, they cannot cause action in isolation. If you ask what action a desire to keep dry will cause, you will get no answer, because that depends on what the agent believes; and similarly for the belief that umbrellas keep you dry. It is only as a pair that a specific action emerges as the rational thing to do: if you want this and you believe that, then (and only then) will an action of a certain type ensue. Actions are selected according to means-end reasoning, and that requires the belief-desire combination. Desires can cause both mental and physical happenings, as when one desire causes another desire or induces a flight of imagination, or when the desire causes the body to react in a certain way (blushing, etc.); but in the case of action only the pair together can produce anything. But then, isn’t there a problem about the causal story, since the components of reasons lack determinate causal powers with respect to action? Normally when two causal factors are at work we have a causal contribution stemming from each, as when two people combine to lift a heavy object: but nothing like this holds of beliefs and desires—considered separately there is no causal input into the motor system. So even if reasons cause actions their components don’t operate in the standard causal manner: there is no cumulative or additive causal contribution. Each is impotent in relation to action, so how can they combine to produce a causal result? We would have a strange kind of causal holism unlike anything else in nature: causation by the whole resulting from non-causation in the parts. Wouldn’t it be better to abandon the idea of rational explanation as causal explanation?

And then there is this familiar point: the holism is also operative across reasons. For an agent will not act on a given reason unless his other reasons are conducive: I might want to keep dry and believe an umbrella will do the trick, but I might also reason that it’s too much trouble to carry an umbrella or that I will look uncool with one or that it’s against my religion to ward off God-given showers. So reasons only cause actions (if they do) against the background of other reasons, but nothing like this holds outside of the rational realm. The causal story thus seems inconsistent with a commonsense form of holism: the alleged causes just don’t operate in the way causes are generally wont to do. The causation is not atomistic (bottom-up) in the standard style–but then why speak of causation at all? The model of specific causal powers possessed by separate states or events breaks down. The causal thesis conflicts with the two kinds of holism described (and generally accepted). There is no causal line linking beliefs and desires separately to specific types of action, so why speak of causality at the level of combinations of the two?

Let me emphasize that this point arises from the logical nature of reasons: the desire component and the belief component are logically incapable of producing a specific action individually. The desire needs an instrumental belief to generate a means, while the belief has no practical consequences without an accompanying desire. Action cannot logically spring from either component alone, but then the model of additive causation breaks down. This is nothing like a combination of separate forces leading to a certain effect. A being without desires will logically never act, and a being with only desires will be bereft of a means of satisfying the desires it has.[3] When we give the reason for which an agent acted we don’t specify two causal factors that combined to produce that action, since neither belief nor desire can cause actions by themselves, or even tend to produce specific actions. This is nothing like explaining why someone has food poisoning by saying that she ate both bad oysters and off chicken. If reasons cause actions, they do so by some magical process that generates holistic causation from atomistic non-causation. Whatever causal powers are possessed by beliefs and desires separately are not exploited in the production of rational action; so it seems merely verbal to insist that causation is operating here as it operates elsewhere. We can call this causation—the concept is capacious—but it is not the kind of causation with which we are familiar in paradigm cases of efficient causation. Certainly it would be futile to seek laws connecting desires with actions or beliefs with actions, as if these could be separate things.

The holism of belief and desire was initially used to refute behaviorism, since no behavioral disposition is associated with a given desire independently of belief (and contrariwise). But the same point undermines the idea that belief and desire operate as summative causal factors in the production of action: no unique disposition to action can be associated with a given belief or desire that might combine with another such disposition. This is a special feature of practical reasoning as means-end reasoning. The type of reasoning that is involved precludes the model of separable causal powers joining forces, because ends need means and means need ends in order to lead to rational action. No causation without causal separation. It is the logic of practical reasoning that precludes the standard model of causal explanation. They were onto something in the 1950s, even though they might have misrepresented what it was.

[1] It was Donald Davidson who mainly instigated this change of view in “Actions, Reasons, and Causes” (1963).

[2] I believe it was Peter Geach who first made this point in Mental Acts (1957).

[3] I hope it is clear that I am not denying that in certain cases beliefs can motivate, as in cognitivist theories of moral motivation. By “desire” I mean (as is standard) any kind of pro-attitude, even if it takes the form of a value judgment; it will still need an instrumental belief to lead to concrete action.

Heat, Color, Shape, and Taste

Galileo’s 1623 discussion of heat and related matters bears revisiting.[1] In it he formulates with particular clarity what later came to be called the primary and secondary quality distinction, using it to address the question of whether motion is the cause of heat. He begins by saying that people labor under the false impression that heat is a “real attribute, property, and quality that truly inheres in the material by which we feel warmed.” (185). He goes on: “Accordingly, I say that as soon as I conceive of a corporeal substance or material, I feel indeed drawn by the necessity of also conceiving that it is bounded and has this or that shape; that it is large or small in relation to other things; that it is in this or that location and exists at this time or that time; that it moves or stands still; that it touches or does not touch another body; and that it is one, a few, or many. Nor can I, by any stretch of the imagination, separate it from these conditions. However, my mind does not feel forced to regard it as necessarily accompanied by such conditions as the following: that it is white or red, bitter or sweet, noisy or quiet, and pleasantly or unpleasantly smelling; on the contrary, if we did not have the assistance of our senses, perhaps the intellect and the imagination by themselves would never conceive of them. Thus, from the point of view of the subject in which they seem to inhere, these tastes, odors, colors, etc., are nothing but empty names; rather they inhere only in the sensitive body, such that if one removes the animal, then all these qualities are taken away and annihilated. However, since we have given them particular names different from those of the primary and real attributes, we have a tendency to believe that these qualities are truly and really different from the primary ones.” (185) He then compares these secondary qualities to the ability of a hand to tickle, remarking that tickling is not an attribute inherent in the hand that tickles; rather, the tickling is “entirely in us” so that “if the animate and sensitive body is removed, it is nothing but an empty name.” (186) He sums up: “I do not believe that in order to stimulate in us tastes, odors, and sounds, external bodies require anything other than sizes, shapes, quantity, and slow or fast motions. I think that if one takes away ears, tongues, and noses, there indeed remain the shapes, numbers, and motions, but not the odors, tastes, or sounds; outside the living animal these are nothing but names, just as tickling and titillation are nothing but names if we remove the armpits and the skin around the nose.” (187)

This distinction, in one form or another, has become extremely familiar to us since Galileo’s time, and has shaped the way we think of perception and reality. It stands opposed to two other theories: (a) that all the perceived qualities of things are primary qualities existing independently of the perceiving subject, and (b) that all the perceived qualities of things are secondary qualities that reflect the inherent constitution of the perceiving subject. Galileo is suggesting a mixed position: qualities divide up into two kinds according to their relation to the perceiving mind, whether within it or without it. Perception partly reflects external reality as it intrinsically is, independently of the mind, and partly reflects the nature of the perceiving subject, having no basis in external reality. It is partly objective and partly subjective. Theory (a) goes along with a realist position on external reality; theory (b) goes along with an idealist position; Galileo’s theory contains both sorts of element, partly realist and partly idealist. The question he doesn’t address, though it is acutely raised by his remarks, is why perception should be thus mixed: why do we (and animals generally) impose these subjective qualities on external things? The question does not arise under theory (a) because all perceived qualities belong to external reality according to that theory, while under theory (b) all our perceptions are manifestations of our own nature. The question for Galileo is this: why is perception partly veridical and partly illusory? We can understand why it should depict reality as it is—that is arguably the point of perceptual representation—but what is the point of presenting us with illusory properties not possessed by external things? Why not be completely objective? Why build error into perception? Why confuse us about what reality is really like?[2] As Galileo says, people are generally wrong about the status of secondary qualities, so they are actively misled about reality: why does perception do that to us? Why are we so deceived by our senses, systematically so? From an evolutionary point of view, shouldn’t such error be selected against, bringing perception more into line with reality? Why does natural selection tolerate our lying eyes? Or: why would God build error so directly into our senses? Descartes’s evil demon may not have total dominion over us, but he seems to have a lot of sway over our powers of perception—he makes us form false beliefs whenever we perceive the world.

A natural answer is that our perception of secondary qualities exists to serve a practical purpose: it records similarities and dissimilarities that are useful to our survival. For example, colors allow us to make finer discriminations and act on them, and tastes and smells enable us to select the right things to eat. No doubt there is something right in this idea, but it faces a couple of queries. First, why not develop senses that directly respond to the objective properties of things, since there is always a primary quality basis for any perceived secondary quality? Why not perceive the actual chemical composition of foodstuffs or the actual wavelengths of light corresponding to different colors? Then the senses would not mislead and would get to the real facts. Second, why do the senses mislead in the way they do—why do they impute the qualities they impose to external objects? Couldn’t they color-code without making objects seem objectively colored? Couldn’t they operate more like pain perception or tickling? They seem to go out of their way to mislead. Locke talked about the possibility of “microscopical eyes”: if we had those we would have no need of secondary qualities, but could limit ourselves to real primary qualities, thus restoring perception to full veridicality. Galileo invented the telescope (or co-invented it), but if he had invented the microscope he might have suggested a way to do away with the perception of secondary qualities. This possibility provides, I believe, the missing part of the answer to our puzzle: the reason we perceive secondary qualities is simply that it is too hard to perceive the operative primary qualities. Notice that the perceived primary qualities are all gross properties, easily detectable by the senses—shape, relative size, etc.—but the primary qualities that underlie secondary qualities are apt to be fine-grained, microscopic, and not readily detectable. How could the tongue explicitly register the chemical composition of a gustatory stimulus? That would be way above its pay grade. How could the eyes deliver perceptions of light waves or photon barrages as such? Instead the mind (the brain) invents qualities to stand in for such imperceptible properties: it conjures up a range of qualities that substitute for the relevant primary qualities. Thus we obtain the advantages of more discriminating perception than can be achieved by the perception of gross primary qualities alone without the burden of penetrating to the hidden microscopic primary qualities of things. And the reason the invented qualities are projected outward is simply that it is simpler that way: you might as well perceive the stimulus as having the property in question given that this is generally the way perception works. If visual stimuli are already perceived as having primary qualities inhering in them objectively, it is less confusing to do the same for secondary qualities—make them also appear to be inherent in external objects. True, a type of perceptual illusion results, but it is not a harmful type of illusion, merely a shortcut to practical efficacy (it is not as if it leads to dangerously false judgments). We can suppose that millions of years ago this apparatus evolved in our fish ancestors and it worked well enough that it persisted in subsequent generations. This is why colors, feels, sounds, smells and tastes evolved—to take advantage of hidden properties without having to represent them directly. They are an economical way to tap into the less accessible parts of reality. God doesn’t need them given his omniscience, but animals must respect the limitations imposed by evolution. So the question raised by Galileo’s distinction can be answered, thus clearing the way for its acceptance.

I wish to observe that some of the senses do not represent primary qualities at all. I do not believe that smell, taste, and hearing represent anything but secondary qualities, while touch and vision do. That is, these senses don’t give us any perception of shape or size or volume or motion or distance; they deal only in the subjective qualities typically assigned to them. They can be supplemented by the other senses, such as touch in the case of taste, but in themselves they do not present any objective qualities of things, only such subjective qualities as sweetness and sourness, pleasant and unpleasant smells, musical tones, etc. With those senses alone we would have no notion of a spatial world, but only a world of subjective qualities. What this tells us is that these senses are concerned exclusively with the inscrutable objective properties that bear on the animal’s life, such as chemical composition and sound waves. By contrast, vision and touch deal with gross properties that can be detected by means of the representational resources of these senses—we can see and feel shape and size. So it is not the case that all senses present a mixed array of properties—some present a purely subjective array (but none present a purely objective array). In a sense, secondary qualities are the natural objects of perception, because they correspond so closely to biological needs without regard for depicting the objective properties of things; shape and size just happen to be objective qualities that bear on the organism’s welfare, while at the same time being gross enough to be perceptible. Taste qualities, by contrast, are not attributable to the mind-independent world, though they correlate with the objective features that provide nutrition (molecular composition ultimately). So in these cases the array of qualities is wholly invented by the mind and imposed on the world. Indeed, it may be argued that even in the case of touch and vision the spatial qualities that are perceived are also really mental constructions, since physics does not recognize such qualities in its ultimate description of things—maybe it’s all strings in a non-Euclidian n-dimensional manifold with no determinate shapes and sizes. It would be surprising if animal perception had the right set of categories for capturing the truly objective nature of reality. But putting that aside, we can say that perceived primary qualities are the exception rather than the rule, so the fact that secondary qualities fail to satisfy veridicality is not really a count against them. They must, however, demonstrate their practical credentials if they are to make their way into our perceptual systems.

I now turn to Galileo’s discussion of heat and motion. His view is that heat is a secondary quality: “I do not believe in the least that besides shape, quantity, motion, penetration, and touch, there is in fire another quality, and that this quality is heat. Rather, I think that heat is in us, so much so that if we remove the animate and sensitive body, heat remains nothing but a simple word.” (188) Thus he declares, “motion is the cause of heat” (189). This conflicts with a well-known doctrine of Kripke’s, namely that heat is identical to molecular motion. Kripke would say that heat (= molecular motion) causes the sensation of heat but it doesn’t cause itself. So motion does not cause heat itself but only the sensation of heat. Yet doesn’t it seem right to say that warmth and coldness are sensory properties of things, properties that our sense of touch can reveal? Couldn’t a plate have the property of being hot without the usual molecular motion? It could certainly cause the sensation of hotness without such motion, so long as the perceiver’s nervous system is hooked up in the right way; and wouldn’t that sensation have a certain sensory quality as its object? The property of being hot seems to be neither molecular motion nor the sensation of being hot: nothing would be hot in a world of molecular motion but no conscious subjects, but it is wrong to identify the quality with the sensation of the quality. I think we need to introduce an extra layer into the story: molecular motion, sensations of heat, and the quality of being hot (or cold). The structure is similar to the case of color: here too we need a threefold division—wavelength of light, sensation of color, and the color itself. The color is not identical to the wavelength but it is also not identical to the sensation of color; it is an extra ontological level. Perhaps we can say that “heat” is ambiguous: sometimes it means molecular motion but sometimes it means the sensible quality we perceive when we have sensations of heat, which is not identical to molecular motion.

Thus Galileo and Kripke can both be right, charitably understood: sometimes “heat” designates molecular motion (in a physics class, say), but sometimes it designates a certain sensory quality that we colloquially refer to with terms like “hot”, “warm”, “lukewarm”, “cool”, and “cold” (in the bath, say). We can reasonably say that the former causes the latter (as well as sensations of the latter), even though in one use “heat” designates molecular motion. Heat (= molecular motion) causes heat (= sensory quality). We might similarly wish to say that “red” is ambiguous, now designating a certain segment of the spectrum and now designating a certain sensory quality—the former being the “cause” (objective basis) of the latter. If we try to work only with motion and sensation, we have trouble explaining the sensation of heat, because the question must arise of what its object is. We can’t say that it is molecular motion because no such concept enters into the nature of the experience; we need something to constitute its “intentional object” (de dicto not de re). This is where hotness comes in: the sensation takes as object a certain quality known through experience, the quality we call “hot”. It is the same for “red”: the intentional object of experience is not the wavelength but a certain irreducible quality that we call “red”. Proceeding from the inside out, there is first the subjective sensation of something being red or hot; then there is the intentional object of this sensation, the quality of being red or hot; and then there is the objective phenomenon that causes the experience to occur (in conjunction with the subject’s nervous system), which might also be referred to in certain contexts with the terms “red” and “heat”. There is more structure here than can be captured by the simple dichotomy of heat (molecular motion) and the sensation of heat. This confuses the discussion and makes the correct position invisible. There is something right in what Galileo says and there is something right in what Kripke says; properly understood their claims are consistent—we just need to recognize the intermediate level of reality (neither subjective sensation nor objective motion). This intermediate level is an invention of the mind and is not to be found in the austere physical description of things, and it exists in order to take advantage of (phenomenologically) undetectable aspects of the physical world. It is tempting to characterize the extra level in dispositional terms: molecular motion has a disposition (power) to cause sensations of heat, as wavelengths of light have a disposition to cause sensations of color. This attempts to do justice to the mind-dependence of these qualities as well as their possession by external things. But we are not required to take the dispositional route; we can regard the properties in question as irreducible primitive properties of things.[3] And that enables us to give a simple and natural account of the content of the relevant experiences: they are experiences as of such irreducible properties (they don’t seem to be as of dispositions to cause experience). So the threefold account provides a pleasing picture of what is going on when a perceiver feels heat or sees color.

Colin McGinn

[1] The text I use is The Essential Galileo, edited by Maurice A. Finocchiaro (Hackett, 2008). The quoted material is from The Assayer.

[2] This question first intrigued me in The Subjective View (Oxford, 1983). Now, nearly forty years later, I have found an answer to it.

[3] I defend this position about color in “Another Look at Color”, reprinted in my book Knowledge and Reality(1999).