Heat, Color, Shape, and Taste
Galileo’s 1623 discussion of heat and related matters bears revisiting. 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? 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. 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.