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Last revised 18 October 2017
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Essay on colour

1. The commonsense view

The phenomenon of colour is a fairly perplexing one. While we know a great deal about it through centuries of inspired scientific detective work, some disconcertingly basic aspects of the phenomenon continue to remain mysterious, shrouded in (as yet) unresolved issues surrounding the larger mind-body problem. In a review of a recent study of colour, writer Malcolm Harris remarks:
To read [this book] as a layman feels like being let in on a shocking secret: Neither scientists nor philosophers know for sure what color is.
—‘Does Color Even Exist?’ (2015), www.newrepublic.com
This sentiment seems to me pretty accurate.

At first sight, colour appears to be a simple property of material things in our environment. We speak of leaves being green and flowers being yellow much as we speak of them having various shapes and sizes. That is to say, like shape and size, we quite naturally take the colours we see to be features of our environment that exist independently of our perceptions of them. Asked to imagine a world devoid of perceivers, for example, I believe that many people would quite naturally imagine it in colour, i.e., just as they would see it. As Colin McGinn says:
… when we see an object as red we see it as having a simple, monadic, local property of the object’s surface. The color is perceived as intrinsic to the object, in much the way that shape and size are perceived as intrinsic.
—‘Another Look at Color’ (1996), p. 132
Alison Simmons puts it this way:
The proposal that there is an ontological difference between a kumquat’s shape and its color does not suggest itself to perceptual experience: both look to be out there in the kumquat.
—‘Perception in Early Modern Philosophy’ (2013), p. 188
That colour is an objective, intrinsic property of the things around us may reasonably be called the commonsense view of colour. This means that the view may fairly be attributed to the average layman—at least before he or she becomes “tainted” by the modern scientific account of colour perception in the brain (see below). Aristotle himself, that ancient embodiment of common sense, seemed to embrace the view, as evinced by his speculations concerning how the eye perceives colour:
If what has colour is placed in immediate contact with the eye, it cannot be seen. Colour sets in movement not the sense organ but what is transparent, e.g. the air, and that, extending continuously from the object to the organ, sets the latter in movement.
De Anima (c. 350 BC), Book 2, Chapter 7
This account of colour perception is no longer credible but Aristotle did seem to accept that colour was an “objective feature” of the world, capable (via the air) of affecting our eyes, whereupon it is seen. Sarah Broadie attests as much:
Aristotle speaks as an unabashed realist about objects of sense. He never questions the existence of what later came to be called the “external” world: a cosmos of physical substances there independently of our knowledge and perception. And he never doubts that the objects composing that world really are as they present themselves to us in sense experience: pungent, fragrant, warm and cold, soft and hard, full of sounds and colors, just as we perceive them to be.
—‘Aristotle’s Perceptual Realism’ (1993), p. 137
The objectivity of colour continued to figure as common sense for more than a thousand years thereafter, as Peter King reminds us:
... during the Middle Ages colors, sounds, and smells were all thought to be unproblematically qualitative features of the external world, no more having a special and inexplicable ‘phenomenal’ dimension than, say, the shape of the chair.
—‘Why isn’t the Mind-Body Problem Medieval?’ (2007), p. 204
So much for a rough statement of the commonsense view of colour. One might no doubt ask further questions of it, e.g., what colour is supposed to be, on this view, over and above something “out there” in the world. When the layman affirms that blood is red, for example, what does he suppose this property of redness to be? What property does he mean to be attributing to blood when he says that blood is red? It took a long time for natural philosophers to make any headway with a question like this (see below); common sense itself has no answer beyond a shrug of the shoulders and the solace that we can recognize the property when we see it.

But never mind this—for the commonsense view of colour is (in any case) very much on the backfoot these days, as most people know. It has come under sustained fire over the past four centuries from various scientific discoveries that seem to suggest that, whatever common sense may aver, there is in fact no such thing as colour (sound, smell, taste, etc.) in the world at large. The world at large consists rather of “qualitatively bland” particles and fields vibrating in space and time – or whatever the latest science says – with no apparent room in it, or even need, for any such quality as colour.

Thus, when you look at a so-called red apple, the apple reflects selected wavelengths of light into your eyes and its “redness” is then generated in your brain as part of your visual experience of the apple. The apple itself is not red in the way you see it to be; the redness is just an illusory product of your mind.

This broad scientific picture is nowadays so familiar and entrenched that one might be surprised to discover that many contemporary philosophers still uphold the commonsense view of colour – the view that colour is “out there” in the world, just as Aristotle, Alhazen and Aquinas supposed it to be. One might well wonder why. Are contemporary philosophers mired in the middle ages, in denial over the findings of enlightenment science? What is the truth about colour anyway? Is it simply “in the mind” or “out there” in the world?

In what follows, I’d like to do three things. The first is to rehearse the broad scientific considerations that have persuaded many people since at least the 17th century that colour is just “in the mind” or is merely “a product of our brains,” as opposed to being “out there” in the world as common sense would have it. This tale is pretty well-worn so I will run through it just to set things up. In recent decades, even more sophisticated considerations about colour-processing in the brain have added fuel to this scientific fire but these additional sophistications (e.g., opponent processing) won’t matter much for my purposes and I will mention them only when they matter.

Second, I’ll explain why, in the face of these considerations, philosophers nevertheless often cling to the commonsense view of colour. Scientists and others often find this hard to understand because philosophers don’t always bother to explain where they are coming from or what they are up to; they also sometimes speak in gnomic terms – see the cartoon. So it may be worth pausing to explain this to the general reader. There is in fact a clear point behind the philosophical resistance to the alleged findings of
science – a point that scientists often seem to be unaware of – but (on the flip side) I will contend also that philosophers, for their part, do not ultimately succeed in rejuvenating the view that colours are “out there” in the world.

To anticipate this last claim a little, a certain familiar difficulty has always bedevilled philosophical attempts to “restore” colours to the external world, viz., that it makes no apparent sense to identify colours (as plain folk know and love them) with physical properties of objects like surface spectral reflectances or microstructural properties, and so on – this being the standard way of “restoring” colours to the external world. The entities being equated seem to belong to completely different categories. No matter how philosophers twist and turn, it seems to me that they are unable to shake off this essential difficulty: in one form or another, it haunts their every attempt to restore colour to the external world. I will illustrate this below with numerous examples of the twisting and turning and I hope that this throws some light on what philosophers are up to because these twists and turns are often unannounced in point of both existence and purpose. My overall conclusion will be that while philosophers are right to resist the scientists’ inclination to relegate colour to the mind, they don’t really have a good way of restoring colour back to the world either. The result is a kind of impasse or aporia in which the place of colour in the natural world is left mysterious. As the Danish physicist Niels Bohr once said in a slightly different context, some essentially new ideas are needed here.

Finally, I’ll say what I think the truth about colour is. At least, I’ll sketch the outlines of an essentially new idea. Whether colour is in the mind or out there in the world is a slightly disingenuous question, of course, since other options may be possible. Some people think of colour as a relation of some sort between mind and world. For example, the redness of an object is simply its disposition to produce certain characteristic kinds of visual sensations in normal human perceivers. On a view like this, the choice is false of whether colour is in the mind or in the world, since, it some sense, it partakes of both. The disadvantage of this approach however is that it continues to run into the familiar difficulty
mentioned above: there is no obvious sense to the claim that colours, as plain folk know them, are just relations or dispositions of a certain kind.

I will propose a completely different way of thinking about colour instead. The colour of an object, I will suggest, has something essentially to do with the way in which the object “stands out” in perception, i.e., with its “perceptual salience,” as I will sometimes say. This notion may seem unfamiliar but many everyday words in our language in fact revolve around it. A bright light, for example, is one which is easily noticed (perceptually very salient) as opposed to a dim one. Likewise, material objects can be bright, luminous or phosphorecent; dark, dull or unreflective. The paint on your walls can be glossy or matt and clothes can be understated or loud. These adjectives apply to everyday things but the point of having them in our language is not so much to characterize the things in themselves as the manner in which they strike us perceptually. There is indeed a diverse family of such words, ranging over all five senses, and functioning in quite different ways – they are not all mere variants of “perceptually conspicuous” and “perceptually inconspicuous,” it should be stressed. A gunshot or a scream may be loud or faint, but it may also be muffled. A star in the sky may be bright or dim, but it may also blink or twinkle. Glass is transparent, which says something not just about the difficulty of seeing it, but also about the ease of seeing what’s behind. Other things are opaque or diaphanous, even invisible. Perfume can be overpowering, food can be tasteless, chameleon-like creatures blend into the background. And so on – it’s a fairly wide-ranging list. I believe that colour terms like ‘red,’ ‘green’ and ‘blue’ belong in this list as well, occupying a special niche of their own. I’ll introduce this novel view fully below and explain why I like it. To see this at once, jump to sections 6-10.

As mentioned, however, I’m going to begin with the broad scientific attack on the commonsense view of colour because everything unfolds pretty naturally from there. I characterized the commonsense view above as holding that colours are objective, intrinsic properties of everyday things in our environment, but I should now mention that some philosophers think that this takes common sense too far. Take (ripe) strawberries, for example. Arguably, all that may safely be attributed to common sense, where colour is concerned, is simply that strawberries are red, i.e., that the statement, ‘Strawberries are red,’ is perfectly true. Common sense need take no further stand on whether the redness is “out there” on the strawberries, or is one of their “intrinsic properties,” or anything remotely metaphysical like that. Here’s a recent expression of a similar point:
... it does not seem so implausible that there is a pan-historically and cross-culturally universal commonsense view that holds that colors are properties of ordinary objects or their surfaces, without making any claims about the perceiver-dependent or independent nature of those properties.
—M. Chirimuuta, Outside Color (2015), p. 32
As a methodological principle, many philosophers think that we should not contradict common sense if we can avoid it – I will explain why below. So a satisfactory account of colour should ideally entail (at least) that strawberries are red (bananas are yellow, grass is green, etc.), just as minimal common sense avers. This “thin” reading of common sense will serve our purposes as well because the broad scientific attack on common sense that we are about to consider does deny even that strawberries are red, and so really does amount to an attack on common sense even thus thinly construed.

2. The early modern view

Some time in the 17th century, natural philosophers began to distinguish the “primary qualities” of material objects from their “secondary” ones.

Very roughly, they suspected that secondary qualities (like colour, sound, smell and taste) were not genuine features of material objects at all, despite initial appearances, but were mere artefacts of sensory perception, misleadingly “projected” by our minds onto the external world. In contrast, primary qualities (like shape, size, motion and number) were deemed to be genuine features of material objects, faithfully represented by our minds in sensory perception.

These must have been extraordinary claims at the time but, over the centuries, this broad scientific view has been gradually developed, come to be widely known, and even passed into popular culture – students no longer bat an eyelid at the suggestion that colour, unlike shape, is “only in the mind.” Indeed, I think it is fair to say that, very broadly speaking, the scientific jury has already made its mind up on this question although, as hinted at above, the philosophical jury is still very much out.

But let’s consider the basic science first.

Writing in 1936, the German psychologist Wolfgang Metzger had this to say about the ordinary attitude towards seeing:
For people who naively look around, their own eyes appear to be a kind of window. As soon as the curtains, the eyelids, are opened there “is” a visible world of things and of other beings out there. Nothing could arouse the suspicion that any of its recognizable properties might originate in the observer or could be codetermined by the observer’s nature – except perhaps for the effects of greater or lesser transparency of the “window panes.” (Wolfgang Metzger, The Laws of Seeing, MIT 2006 English translation, Introduction.)
In this passage, Metzger acknowledges the “transparency” or “diaphanousness” of seeing (as philosophers often call it) but regards it as a potentially misleading thing. In particular, it makes us overlook the possibility that we do not see things as they are “in themselves,” but rather contribute ourselves in some way (by the very act of seeing them) towards their appearances. Metzger was thinking inter alia about the feature of colour:
Colors are now recognized as the result of external influences on particular parts of the human body. They are called “sensations” in order to express that they belong together with heat or pressure, which one really feels on one’s own body when touching a hot iron or carrying a heavy sack on one’s back. Nevertheless it is strange that these odd sensations stubbornly seem to cling to the external surfaces of objects and cannot, not even with considerable effort, be sensed (like the pain of blinding light) as within the eyes. But in view of the incontrovertible evidence of physics, we cannot be fooled into thinking that light rays (and even light sources) are themselves colored. They only evoke the experience of certain colors “within” us depending on their wavelength, and therefore the colors themselves are a property of the human nature (i.e., the brain, not the external world). (Ibid.)
Metzger was writing in the twentieth century but this way of thinking about colour goes back to Galileo in the seventeenth, who was one of the first great scientists to articulate the view:
I think that tastes, odors, colors, and so on are no more than mere names so far as the object in which we place them is concerned, and that they reside only in the consciousness. Hence if the living creature were removed, all these qualities would be wiped away and annihilated. (The Assayer 1623, trans. Stillman Drake, online copy here.)
The broad idea is that colour is “in the mind” and not really a property of the things in our environment, as is natural to think. Alternately, it is said that colour is in the consciousness, or in the head, or in the brain, or even in the eyes, and so on. (See later. I will say “in the mind” for now.) Similar sentiments may be found among many other early modern philosophers from Descartes and Newton to Locke and Hume, and the view continues to be found in contemporary psychology texts. Here is psychologist Stephen Palmer in a work from 1999:
People universally believe that objects look colored because they are colored, just as we experience them. The sky looks blue because it is blue, grass looks green because it is green, and blood looks red because it is red. As surprising as it may seem, these beliefs are fundamentally mistaken. Neither objects nor lights are actually ‘colored’ in anything like the way we experience them. Rather, color is a psychological property of our visual experiences when we look at objects and lights, not a physical property of those objects or lights. The colors we see are based on physical properties of objects and lights that cause us to see them as colored, to be sure, but these physical properties are different in important ways from the colors we perceive. (Vision Science: Photons to Phenomenology, MIT Press 1999, p. 95.)
One reason why scientists tend to uphold a view of this sort is that they seem to have no use for any such property as “colour” (as we normally understand this) in explaining how the inanimate world works. The inanimate world, in the eyes of science, is a world of myriad moving lifeless things of different shapes, sizes and other such “primary qualities.” Various laws govern how they interact with one another but no law ever turns on the “colour” of anything. For example, the law of gravity says that the force with which two bodies attract each other depends on their masses and distance apart, but hardly on their colour. (And so on – no law of physics ever mentions the property of colour.)

Even in cases where colour might at first seem to figure in a scientific generalization, the reference to colour turns out to be otiose. Black things might heat up faster in the sun than white ones but this can be explained without mentioning their colours at all but by their relative tendencies to absorb or reflect certain kinds of radiation. Here’s another example from D. M. Armstrong:
Does the high-pitched sound shatter the glass? To superficial observation it might appear to do so. But investigation reveals that it is vibrations in the air which really break the glass. (‘Smart and the Secondary Qualities.’)
As far as science is concerned, in other words, there might as well be no such thing as colour (sound, smell, etc.) in the world “out there.” Sydney Shoemaker sums this up very well:
Since the 17th century, a central issue in metaphysics and epistemology has concerned the status of what, following John Locke, have come to be called “secondary qualities”: colors, odors, tastes, sounds, warmth and coldness, etc. The problem is posed by the fact that while these qualities are experienced as belonging to objects in our external environment – the apple is experienced as red, the rose as fragrant, the lemon as sour, etc. – the scientific world-view, as developed in the 17th century and subsequently, seems to allow no place for them among the objective properties of material bodies. A common solution has been to deny, in one way or another, that these qualities do objectively inhere in external things in the way we experience them as doing. Galileo seems to have held that colors, etc., are properties of our sensations rather than of external things. (‘Qualities and Qualia: What’s in the Mind?,’ Philosophy and Phenomenological Research, vol. 50, 1990, opening paragraph.)
Indeed, where science is concerned, colour needs to come into it only when sentient beings enter the picture. Put a man in a field and you now need to explain why he sees the field as being green. Without the man’s act of seeing, the “greenness” of the field would never get a mention. Colour makes an appearance only when mind encounters world, but not before. And so it is natural to think that colour has something to do with the way in which the mind “receives” or “represents” the world and is not a feature of the external world as such, i.e., the world apart from the mind.

As mentioned previously, this kind of picture has now infiltrated popular culture to the extent of threatening to be the new common sense. I don’t think it has quite reached that stage but it is getting pretty close. Roughly speaking, colourless things out there in the world send signals to our brains (through our eyes) and colours are then conjured up in our brains which we mistakenly project onto the things that we see. The colours are only ever “in our minds” but an unthinking person (the common man) may fail to realize this. This is the broad picture.

It is worth comparing this early modern conception of colour perception to the Aristotelian/medieval conception that preceded it. Aristotle’s conception was briefly mentioned above:
Colour sets in movement not the sense organ but what is transparent, e.g. the air, and that, extending continuously from the object to the organ, sets the latter in movement.



Descartes












There are two basic reactions one might have to this broad picture. Some consider it a profound discovery of science that fully deserves to be passed down to the common man. This is no doubt the view of Palmer, quoted above, and numerous others eminent scientists as well. Here’s a similar sentiment from neurologist Semir Zeki, quoted in Byrne & Hilbert, ‘Color realism and color science’ (2003):
The results described here ... suggest that the nervous system, rather than analyze colours, takes what information there is in the external environment, namely, the reflectance of different surfaces for different wavelengths of light, and transforms that information to construct colours, using its own algorithms to do so. In other words, it constructs something which is a property of the brain, not the world outside. (‘Color coding in the visual cortex’ 1983, p. 764.)
Others however consider this broad picture to be a piece of misguided metaphysics amounting to a pretty serious error. For those in the second camp – and here reside many contemporary philosophers – the following quote captures something of the dismay and despair:
According to projectivists, science has revealed something very important about colours, something which used to be understood clearly, but which recent philosophers have failed to grasp. People with colour vision represent the world to themselves in a specific way. They represent the world as containing objects, spatially separated from us, which have various different colours. Projectivists say that when people represent things to themselves that way, they are misrepresenting the world. Material objects do not have the colour properties we represent them as having. [Footnote: According to the projectivists, Galileo, Newton, Locke, and others were right about colours. Armstrong, Smart, Shoemaker, Peacocke, McDowell, Wiggins, McGinn, and many others among our contemporaries are all wrong.] (Bigelow, Collins & Pargetter, ‘Colouring in the World.’)
The contemporary philosophers cited here (Armstrong, Smart, Shoemaker, Peacocke, ...) all believe that colours are genuine features of the world at large, siding largely with common sense on this score against the likes of Palmer and Zeki above. They disagree over some details—see below—but they are united in rejecting the broad Galilean idea that colours are mere “projections” of the mind onto the external world, i.e., features that the mind “reads into” the world that are not really there. The Galilean idea may draw some superficial support from science, they would concede, but, over the centuries, philosophers have learnt that the idea is not as easy to sustain as one might first think. In fact, it is the thin edge of that wedge known nowadays as the “hard problem of consciousness.” And so it behooves us to consider whether colours might not be better reinstated in the external world, where they seemed in the first place to be.—This is the broad stance.

To fully appreciate this point of view, we need to unravel a few historical threads that are often glossed over in the contemporary philosophical literature on colour – possibly because they are regarded as well-known. It is convenient to start with the writings of the 17th-century English philosopher John Locke, who lived in the same era as Galileo, Descartes, Boyle and Newton, and was deeply immersed in the revolutionary scientific discoveries of his age, while being an out-and-out philosopher to boot. His pregnant remarks in his Essay Concerning Human Understanding (1690) on the “secondary qualities” (including colour) were among the most sophisticated of his time and have given philosophers plenty to chew on since then.










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1. The commonsense view

The phenomenon of colour is a fairly perplexing one. Although we know a great deal about it through centuries of heroic scientific detective work, some rather basic aspects of the phenomenon continue to remain mysterious, shrouded as they are in larger, unresolved issues surrounding the perennial mind-body problem. (See below.)

At first sight, colour appears to be a simple (monadic) property of material things in our environment. We speak of leaves being green and buses being yellow much as we speak of them as being thusly shaped, sized or numbered. Indeed, like shape, size and number, we quite naturally take the colours we see to be features of our environment that exist independently of our perceptions of them. Asked to imagine a world devoid of perceivers, for example, I believe that many people would quite naturally imagine it in colour, i.e., just as they would see it. Colin McGinn puts it this way:
... when we see an object as red we see it as having a simple, monadic, local property of the object’s surface. The color is perceived as intrinsic to the object, in much the way that shape and size are perceived as intrinsic. No relation to perceivers enters into how the color appears; the color is perceived as wholly on the object, not as somehow straddling the gap between it and the perceiver. (‘Another Look at Color,’ Journal of Philosophy, vol. 93, 1996.)
Alison Simmons uses a more exotic example:
The proposal that there is an ontological difference between a kumquat’s shape and its color does not suggest itself to perceptual experience: both look to be out there in the kumquat. (‘Perception in Early Modern Philosophy,’ p. 188.)
This ordinary attitude towards colour may be mistaken – or not; see below – but it is certainly understandable. For, whenever we see, we normally just “take in” whatever we see, paying scant attention to the fact that we are engaged in an act of seeing. Our attention is normally focused on the things that we see, rather than on the fact that we see them. Indeed, we tend to regard the act of seeing itself as doing little more than “enabling” us to know various aspects of our environment (which is what usually interests us). In particular, under good viewing conditions, we do not imagine that the act of seeing in any way distorts the nature of whatever we see, in the way that stepping into a puddle to gauge its depth (say) might well alter that depth. And so when we see a coloured thing, we naturally take the colour we see to be an intrinsic feature of the thing – the act of seeing the thing, we are apt to suppose, simply “lets us know” what colour it is.

Some philosophers speak in this vein of the “transparency” or “diaphanousness” of seeing, often meaning thereby to endorse the ordinary attitude towards seeing just described. Where colour is concerned, in particular, these philosophers are apt to agree with the common man that our physical environment is coloured in pretty much the way we see it to be. Michael Watkins expresses this vividly in his book, Rediscovering Colors: A Study in Pollyanna Realism:
Pollyanna, the heroine of my story, endorses a simple theory. It claims that objects are colored just as they are shaped; typically, objects appear to have the colors they have because of the colors they have; objects were colored before there were observers to perceive those colors; and objects have their colors in the dark, when no one is watching, and sometimes (though not usually) when they appear to have some color they do not have. Colors, Pollyanna believes, are intrinsic, observer-independent, properties of objects that are causally responsible for objects appearing colored. Colors, in other words, are just as Pollyanna experiences them most of the time. Pollyanna doesn’t have much to say about colors. She doesn’t worry about the ontological status of colors, or whether objects are really colored, or whether colors are properties of objects or only “in her head.” Why worry? The answers seem obvious. Pollyanna can see that objects are colored and she can see that they are really colored too. Indeed, Pollyanna can’t help but believe that objects are colored and she suspects that’s true of everyone else (although she acknowledges, of course, that some believe that they don’t believe it). Once upon a time, almost everyone agreed with Pollyanna. Those were happy days, before the dark days of scientism, before philosophers lost their metaphysical nerve. (Rediscovering Colors: A Study in Pollyanna Realism, Springer 2002, pp. 1-2.)
As Watkins laments, however, a commonsense view of this sort has found itself increasingly on the backfoot over the centuries. It has come under attack primarily from various scientific discoveries which seem to suggest that there is no such thing as colour in the world at large. The world at large is supposed to consist rather of nothing but particles and fields vibrating in space and time – or something like that, depending on the latest science – with no apparent room in it (or even need) for anything like colour. A scientific picture of this sort is nowadays so familiar that philosophers who continue to defend the commonsense view of colour may be suspected of being in denial over the findings of science. Worse, these philosophers also squabble over the “correct way” to defend the commonsense view and this is also part of Watkins’ lament above, for his way of defending the view is considered even in philosophical circles to be particularly audacious, as we will see.

In what follows, I would like to do two things. The first is to explain why philosophers often tend to defend the commonsense view of colour in the face of the apparently strong scientific case to the contrary.
Scientists (and other interested observers) sometimes find this hard to understand because philosophers don’t always bother to explain (except in “insider’s language”) where they are coming from and what they are up to. So I think it may be worth pausing to explain in somewhat plainer English what these philosophers take themselves to be doing. I will also explain some of their internal squabbles over “how best” to defend the commonsense view.

For better or worse, I will not cover this in great detail. The philosophical literature on colour is vast and confusing and there is no real point to scouring it in detail. I will say enough to enable a scientist (say) to make substantial sense of it, and also enough to justify a certain contention of mine, viz., that while much of this literature is interesting in its own right, it also tends (disappointingly) to bypass the really “hard issue” when it comes to the phenomenon of colour. The hard issue is essentially that of understanding where colour fits into the natural world. Our current scientific worldview seems to have no natural home for colour, so what is this thing we call colour and where does it fit into the overall scheme of things? Note that the issue is not resolved simply by declaring, as a scientist might, that colour is a sensory illusion that exists “only in the mind” – for what is the mind and what is it for something to exist “in” the mind? If you look inside the brain, for example, you won’t find any colours there, so what are these colours and where are they hiding? This is one place where philosophers are one step ahead of scientists because they more clearly see the difficulties involved in trying to “relegate” colour to the mind. Scientists tend to assume that they will eventually figure out the mind and sort this residual issue out but philosophers have had their eye on this “residual issue” for a long time now and know it to be a bit of a Gordian knot.

So when philosophers talk about colour, they are often wrestling with this Gordian knot – the “hard issue” of colour; the issue of finding a comfortable home for colour in the natural world. At least, that’s one of the things they are supposed to be doing, whatever else they may have to say about colour. But, as I said, their writings on this score often tend to disappoint. Many philosophers of colour give every appearance of addressing the hard issue without in my opinion really doing so. All too often, certain (related) “easy questions” are raised and addressed while the really “hard question” is skirted. A similar thing happens with philosophical work on consciousness, as is well-known. David Chalmers famously remarked that much of this work addresses the “easy problems” of consciousness without tackling the really “hard” one. I believe that something similar is true of much philosophizing about colour. It is easiest to make this charge out in the same breath as explaining the specific moves that philosophers make when they talk about colour, so that’s what I’ll do below. (See sections 2 to 5.)

The second thing I will do is to set the philosophical literature aside and grapple directly with the hard issue myself. I will suggest a relatively novel way of “locating” colour in the natural world that is not found in the literature. It is not found there partly because, as mentioned, the literature tends to skirt the hard issue, but also (I think) because it is not an approach that comes readily to mind – it is not the first thing you would think of. Briefly, however, there is an offbeat way of approaching the phenomenon of colour that has largely been overlooked but it is of a sort that I think we need to consider if we are to properly meet the challenge of finding a place for colour in the overall scheme of things. (See sections 6 to 10.)

We should begin however with the broad scientific attack on the commonsense view of colour because everything unfolds pretty naturally from there. Some time in the 16th century, natural philosophers began to distinguish the “primary qualities” of material objects from their “secondary” ones. Very roughly, they suspected that the secondary qualities (like colour, sound, smell and taste) were not genuine features of material objects at all, despite initial appearances, but were mere artefacts of sensory perception, misleadingly “projected” by our minds onto the external world. In contrast, the primary qualities (like shape, size, motion and number) were deemed to be genuine features of material objects, faithfully represented by our minds in sensory perception. These must have been extraordinary claims at the time but, over the centuries, this broad scientific view has been gradually developed, come to be widely known, and even passed into popular culture – students no longer bat an eyelid at the suggestion that colour, unlike shape, is “only in the mind.” Indeed, I think it is fair to say that, very broadly speaking, the scientific jury has already made its mind up on this question although, as hinted at above, the philosophical jury is still very much out.

But let’s consider the basic science first and save the philosophy for later.

2. The early modern view

Writing in 1936, the German psychologist Wolfgang Metzger had this to say about the ordinary attitude towards seeing:
For people who naively look around, their own eyes appear to be a kind of window. As soon as the curtains, the eyelids, are opened there “is” a visible world of things and of other beings out there. Nothing could arouse the suspicion that any of its recognizable properties might originate in the observer or could be codetermined by the observer’s nature – except perhaps for the effects of greater or lesser transparency of the “window panes.” (Wolfgang Metzger, The Laws of Seeing, MIT 2006 English translation, Introduction.)
In this passage, Metzger acknowledges the “transparency” or “diaphanousness” of seeing (discussed briefly in the previous section) but regards it as a potentially misleading thing. In particular, it makes us overlook the possibility that we do not see things as they are “in themselves,” but rather contribute ourselves in some way (by the very act of seeing them) towards their appearances. Metzger was thinking inter alia about the feature of colour:
Colors are now recognized as the result of external influences on particular parts of the human body. They are called “sensations” in order to express that they belong together with heat or pressure, which one really feels on one’s own body when touching a hot iron or carrying a heavy sack on one’s back. Nevertheless it is strange that these odd sensations stubbornly seem to cling to the external surfaces of objects and cannot, not even with considerable effort, be sensed (like the pain of blinding light) as within the eyes. But in view of the incontrovertible evidence of physics, we cannot be fooled into thinking that light rays (and even light sources) are themselves colored. They only evoke the experience of certain colors “within” us depending on their wavelength, and therefore the colors themselves are a property of the human nature (i.e., the brain, not the external world). (Ibid.)
This way of thinking about colour goes back to Galileo, who was one of the first great scientists to articulate the view:
I think that tastes, odors, colors, and so on are no more than mere names so far as the object in which we place them is concerned, and that they reside only in the consciousness. Hence if the living creature were removed, all these qualities would be wiped away and annihilated. (The Assayer 1623, trans. Stillman Drake, online copy here.)
The broad idea is that colour is “in the mind” and not really a property of the things in our environment, as is natural to think. Alternately, it is said that colour is in the consciousness, or in the head, or in the brain, or even in the eyes, and so on. (See later. I will say “in the mind” for now.) Similar sentiments may be found among many other early modern philosophers from Descartes and Newton to Locke and Hume, and the view continues to be found in contemporary psychology texts. Here is psychologist Stephen Palmer in a work from 1999:
People universally believe that objects look colored because they are colored, just as we experience them. The sky looks blue because it is blue, grass looks green because it is green, and blood looks red because it is red. As surprising as it may seem, these beliefs are fundamentally mistaken. Neither objects nor lights are actually ‘colored’ in anything like the way we experience them. Rather, color is a psychological property of our visual experiences when we look at objects and lights, not a physical property of those objects or lights. The colors we see are based on physical properties of objects and lights that cause us to see them as colored, to be sure, but these physical properties are different in important ways from the colors we perceive. (Vision Science: Photons to Phenomenology, MIT Press 1999, p. 95.)
One reason why scientists tend to uphold a view of this sort is that they seem to have no use for any such property as “colour” (as we normally understand this) in explaining how the inanimate world works. The inanimate world, in the eyes of science, is a world of myriad moving lifeless things of different shapes, sizes and other such “primary qualities.” Various laws govern how they interact with one another but no law ever turns on the “colour” of anything. For example, the law of gravity says that the force with which two bodies attract each other depends on their masses and distance apart, but hardly on their colour. (And so on – no law of physics ever mentions the property of colour.)

Even in cases where colour might at first seem to figure in a scientific generalization, the reference to colour turns out to be otiose. Black things might heat up faster in the sun than white ones but this can be explained without mentioning their colours at all but by their relative tendencies to absorb or reflect certain kinds of radiation. Here’s another example from D. M. Armstrong:
Does the high-pitched sound shatter the glass? To superficial observation it might appear to do so. But investigation reveals that it is vibrations in the air which really break the glass. (‘Smart and the Secondary Qualities.’)
As far as science is concerned, in other words, there might as well be no such thing as colour (sound, smell, etc.) in the world “out there.” Sydney Shoemaker sums this up very well:
Since the 17th century, a central issue in metaphysics and epistemology has concerned the status of what, following John Locke, have come to be called “secondary qualities”: colors, odors, tastes, sounds, warmth and coldness, etc. The problem is posed by the fact that while these qualities are experienced as belonging to objects in our external environment – the apple is experienced as red, the rose as fragrant, the lemon as sour, etc. – the scientific world-view, as developed in the 17th century and subsequently, seems to allow no place for them among the objective properties of material bodies. A common solution has been to deny, in one way or another, that these qualities do objectively inhere in external things in the way we experience them as doing. Galileo seems to have held that colors, etc., are properties of our sensations rather than of external things. (‘Qualities and Qualia: What’s in the Mind?,’ Philosophy and Phenomenological Research, vol. 50, 1990, opening paragraph.)
Indeed, where science is concerned, colour needs to come into it only when sentient beings enter the picture. Put a man in a field and you now need to explain why he sees the field as being green. Without the man’s act of seeing, the “greenness” of the field would never get a mention. Colour makes an appearance only when mind encounters world, but not before. And so it is natural to think that colour has something to do with the way in which the mind “receives” or “represents” the world and is not a feature of the external world as such, i.e., the world apart from the mind.

As mentioned previously, this kind of picture has now infiltrated popular culture to the extent of threatening to be the new common sense. I don’t think it has quite reached that stage but it is getting pretty close. Roughly speaking, colourless things out there in the world send signals to our brains (through our eyes) and colours are then conjured up in our brains which we mistakenly project onto the things that we see. The colours are only ever “in our minds” but an unthinking person (the common man) may fail to realize this. This is the broad picture.

There are two basic reactions one might have to this broad picture. Some consider it a profound discovery of science that fully deserves to be passed down to the common man. This is no doubt the view of Palmer, quoted above, and numerous others eminent scientists as well. Here’s a similar sentiment from neurologist Semir Zeki, quoted in Byrne & Hilbert, ‘Color realism and color science’ (2003):
The results described here ... suggest that the nervous system, rather than analyze colours, takes what information there is in the external environment, namely, the reflectance of different surfaces for different wavelengths of light, and transforms that information to construct colours, using its own algorithms to do so. In other words, it constructs something which is a property of the brain, not the world outside. (‘Color coding in the visual cortex’ 1983, p. 764.)
Others however consider this broad picture to be a piece of misguided metaphysics amounting to a pretty serious error. For those in the second camp – and here reside many contemporary philosophers – the following quote captures something of the dismay and despair:
According to projectivists, science has revealed something very important about colours, something which used to be understood clearly, but which recent philosophers have failed to grasp. People with colour vision represent the world to themselves in a specific way. They represent the world as containing objects, spatially separated from us, which have various different colours. Projectivists say that when people represent things to themselves that way, they are misrepresenting the world. Material objects do not have the colour properties we represent them as having. [Footnote: According to the projectivists, Galileo, Newton, Locke, and others were right about colours. Armstrong, Smart, Shoemaker, Peacocke, McDowell, Wiggins, McGinn, and many others among our contemporaries are all wrong.] (Bigelow, Collins & Pargetter, ‘Colouring in the World.’)
The contemporary philosophers cited here (Armstrong, Smart, Shoemaker, Peacocke, ...) all believe that colours are genuine features of the world at large, siding largely with common sense on this score against the likes of Palmer and Zeki above. They disagree over some details—see below—but they are united in rejecting the broad Galilean idea that colours are mere “projections” of the mind onto the external world, i.e., features that the mind “reads into” the world that are not really there. The Galilean idea may draw some superficial support from science, they would concede, but, over the centuries, philosophers have learnt that the idea is not as easy to sustain as one might first think. In fact, it is the thin edge of that wedge known nowadays as the “hard problem of consciousness.” And so it behooves us to consider whether colours might not be better reinstated in the external world, where they seemed in the first place to be.—This is the broad stance.

To fully appreciate this point of view, we need to unravel a few historical threads that are often glossed over in the contemporary philosophical literature on colour – possibly because they are regarded as well-known. It is convenient to start with the writings of the 17th-century English philosopher John Locke, who lived in the same era as Galileo, Descartes, Boyle and Newton, and was deeply immersed in the revolutionary scientific discoveries of his age, while being an out-and-out philosopher to boot. His pregnant remarks in his Essay Concerning Human Understanding (1690) on the “secondary qualities” (including colour) were among the most sophisticated of his time and have given philosophers plenty to chew on since then.

3. Locke on colour

John Locke (1632-1704) is often regarded as one of the great enlightenment thinkers. He taught himself medicine, wrote widely on economics and politics, epistemology and the philosophy of mind – and, where the specific issue of secondary qualities was concerned, accepted and championed the broad Galilean outlook described in the previous section.

Locke’s remarks on colour and the other secondary qualities are found in chapter 8 of book 2 of his Essay, a chapter innocently titled, ‘Some further considerations concerning our simple ideas.’ This chapter contains twenty-six short sections and is well-known to philosophers of colour, who have pored over Locke’s words with a fine-toothed comb. That Locke accepted the broad Galilean outlook is not in doubt and evident from passages like these:
What I have said concerning colours and smells may be understood also of tastes and sounds, and other the like sensible qualities; which, whatever reality we by mistake attribute to them, are in truth nothing in the objects themselves, but powers to produce various sensations in us, and depend on those primary qualities, viz. bulk, figure, texture, and motions of parts; as I have said. (Section 14.)

The particular bulk, number, figure, and motion of the parts of fire, or snow, are really in them, whether any one’s senses perceive them or no; and therefore they may be called real qualities, because they really exist in those bodies: but light, heat, whiteness or coldness, are no more really in them, than sickness or pain is in manna. Take away the sensation of them; let not the eyes see light, or colours, nor the ears hear sounds; let the palate not taste, nor the nose smell; and all colours, tastes, odours, and sounds, as they are such particular ideas, vanish and cease, and are reduced to their causes, i.e. bulk, figure, and motion of parts. (Section 17.)
For all the brilliance of Locke and the early moderns, however, the idea of evicting colours, sound and smells (etc.) from the material world and relocating them to the mind has proven over time to be rather difficult to sustain. Locke often spoke of colours, sounds and smells as being “in us,” as did Galileo and Descartes. They did not mean, of course, literally in us, i.e., inside our bodies, along with our hearts and spleens. These entities were supposed rather to be “sensations” or “ideas,” with attendant mental connotations. They were also said to be “in the mind” or “in the consciousness” and the like. Malebranche spoke of them as being in the soul:
Our eyes represent colors to us on the surface of bodies and light in the air and in the sun; our ears make us hear sounds as if spread out through the air and in the resounding bodies; and if we believe what the other senses report, heat will be in fire, sweetness will be in sugar, musk will have an odor, and all the sensible qualities will be in the bodies that seem to exude or diffuse them. Yet it is certain ... that all these qualities do not exist outside the soul that perceives them ... (Elucidations, 6.)
The language here is not crucial since the underlying conceptual apparatus was relatively new and being developed as they wrote. (The old scholastic or Aristotelian ways of speaking were gradually being displaced.) This well-known passage from Descartes expresses the same embryonic conception:
It is clear then that when we say we perceive colors in objects, it is really just the same as saying that we perceive in objects something as to whose nature we are ignorant but which produces in us a very clear and vivid sensation, what we call the sensation of color. (Principles of Philosophy, para. 70.)
The trouble arises when we try to get clearer on what it could mean to say that colours, sounds and smells reside “within us.” Here, the early moderns seemed to do little better than treat the mind or the soul as an “immaterial realm” capable of housing entities that seemed out of place in the material world. External physical impulses, transmitted via our sense organs into our skulls, conspired somehow to generate sensations of colour, sound and smell in this convenient immaterial realm. Or, in another version, the physical impulses were said to correlate with the occurrence of these sensations in the immaterial realm, e.g., through the grace of God. Bells and whistles notwithstanding, the essential early modern account never really got much better than this. Most of the early moderns were immaterialists about the mind in this sketchy way and seemed content to trust that future developments would clarify the nature of immaterial minds and their relation to material bodies. A notable exception was Thomas Hobbes, who thought that such talk of immaterial minds bordered on the nonsensical.

Five hundred years on, Hobbes seems to have been vindicated, and it is no longer credible to think of the mind – assuming this concept is still useful – in the early modern way. If anything, the mind is more credibly identified nowadays with the physical brain, somewhat as Hobbes suspected, although this view has difficulties of its own (see below). We haven’t quite deciphered the mind and its place in nature, nor even the specific phenomenon of sense perception, but it does increasingly seem that any broad “dualism” on which colours, sounds and smells are evicted from the material world and housed in an immaterial realm is not a likely contender. The early moderns cannot reasonably have foreseen this as they had a lot on their plate – they were not just endeavouring to develop a new way of thinking about sense perception but were also busy keeping up with the latest discoveries in science such as the discovery of the retina and the nature of light.

There is another difficulty that they did foresee, however, and which has proven to be the deeper one.

On the Galilean view, as mentioned, colourless things out there in the world send signals to our brains (through our eyes) and colours are then conjured up in our immaterial minds, which we mistakenly project onto the things that we see. But how does a physical signal transform itself into an immaterial sensation? In the case of sound, Galileo says:
Sounds are made and heard by us when the air—without any special property of “sonority” or “transonority”—is ruffled by a rapid tremor into very minute waves and moves certain cartilages of a tympanum in our ear. External means capable of thus ruffling the air are very numerous, but for the most part they may be reduced to the trembling of some body which pushes the air and disturbs it. Waves are propagated very rapidly in this way, and high tones are produced by frequent waves and low tones by sparse ones.
This is a promising start at a mechanical explanation but notice that Galileo says nothing about how frequent or sparse waves (in the physical ear) are able to produce high or low tones (in the immaterial mind). No doubt, he believed that the answer would be revealed in time but there is a clear difficulty here about what is supposedly happening at the interface between body and mind, where material impulses are transformed into immaterial sensations.

Half a century later, Isaac Newton had discovered the refraction of light and the associated colours of the rainbow – a connection that Galileo knew nothing of – but he too professed ignorance over how “phantasms” of colour are generated in our minds:
But, to determine more absolutely, what Light is, after what manner refracted, and by what modes or actions it produceth in our minds the Phantasms of Colours, is not so easie. (‘New Theory about Light and Colors’ 1672, online copy here.)
Locke himself was certainly aware of the difficulty:
We are so far from knowing what figure, size, or motion of parts produce a yellow colour, a sweet taste, or a sharp sound, that we can by no means conceive how any size, figure, or motion of any particles, can possibly produce in us the idea of any colour, taste, or sound whatsoever; there is no conceivable connexion betwixt the one and the other. (Essay, Book 4, Chapter 3, Section 13.)
Of all the difficulties faced by the early moderns in their attempts to make sense of sense-perception, this one has persisted to this day. Nowadays known as the “hard problem of consciousness,” it is not resolved simply by replacing the notion of an immaterial mind with a physically more respectable conception. If anything this makes things harder. Thus, when you see a red object (say), physical waves are transmitted to your eyes and brain, and countless neurons fire in your visual cortex – and then what? Few would nowadays say that a “red sensation” is then generated in your “immaterial mind,” but one struggles to find a better story. Ever since the 17th century, we have known that activity in the brain causes us to “see red,” but how this phenomenon of “seeing red” is supposed to fit within the brain remains a baffling question. An immaterial mind at least sounds like the right sort of thing in which an episode of “seeing red” might be generated and housed, but how do you make sense of the phenomenon of “seeing red” if you think that it occurs inside the head but reject the idea of an immaterial mind? Hobbes himself, arch materialist, did no better than Galileo, Newton or Locke in explaining what was going on when colours, sounds and smells are generated “within us.” He writes, in the case of sound:
... the clapper hath no sound in [the bell], but motion, and maketh motion in the internal parts of the bell; so the bell hath motion, and not sound, that imparteth motion to the air; and the air hath motion, but not sound; the air imparteth motion by the ear and nerve unto the brain; and the brain hath motion but not sound ...
... from the brain, [the motion] reboundeth back into the nerves outward, and thence it becometh an apparition without, which we call sound. (Hobbes 1658, II.9, quoted in Alison Simmons, ‘Perception in Early Modern Philosophy.’)
As Simmons comments, “It’s not at all clear that this is an improvement on the dualist’s response.”

The difficulty that the early moderns faced thus remains very much alive, only transmuted into a different form. Indeed, contemporary materialists have not progressed very far beyond Hobbes. And here we encounter one of the main reasons why contemporary philosophers are suspicious of the view that colours (sounds, smells, etc.) are all “in the head.” For those who propound this doctrine usually either mean something like “in the mind,” with intimations of immateriality lurking not far behind, or else they are unable to say what exactly they mean by “in the head.” They are simply gesturing at some embryonic conception of what is going on in the way of the early moderns. If we accept that there are no such things as immaterial minds and that the only things within our heads are our brains, then the claim that colours, sounds and smells are “in the head” is considerably less attractive – whatever could you possibly mean?

As mentioned, philosophers are keenly aware of this difficulty and it vividly informs their reluctance to accept the standard sketch often found in psychology texts that colours are “generated in the brain.” It is difficult to know what scientists think of this difficulty but their attitude towards it often seems to philosophers to verge on the cavalier:
We know from psychophysical and neurophysiological investigations that color is created somewhere in the brain, although the exact location of this process is still unknown, and we even have no idea what entities the sensations called color are. Because the scientific and philosophical discussion of color should not go beyond our actual knowledge ... it is not at all a matter of taste or philosophical viewpoint where colors can be thought to be located. In short, colors appear only at a first (naive) glance to be located in objects; closer inspection shows that colors are produced in the brain ... (Backhaus & Menzel 1992, p. 28)
This is what philosophers call a “promissory note,” i.e., a blank cheque that nobody quite knows how to cash. Scientists sometimes say – as is implicit in the remarks above – that it is just a matter of time before we figure out what these “colour sensations” are and how they are created in the brain, but this plea of ignorance can only last so long before it starts to wear thin – and it has been 500 years.

When contemporary philosophers write about colour, these considerations are in the back of their minds. And this is one reason why they are often prepared to reconsider what science has supposedly shown us about colour. Consider the general stance of David Armstrong, captured here by his former student Keith Campbell:
Armstrong’s thought about colour has been conditioned not only by the search for system; it has been made more difficult by a further condition that he has always imposed: The system may admit of no nonmaterial realm of concrete substances. So the classic Lockean strategy, that of providing a refuge for colour in the immaterial perceiving minds of creatures with colour vision, has never been a serious option for him ... It is not that Armstrong does not believe in minds. On the contrary, he holds that minds are substances we know to exist. The problem is that, with minds being functional systems whose categorical basis consists in nervous tissue, there is no more comfortable place for the colours in the brain than there is on the surfaces of the material objects that seem to bear them. (Keith Campbell, ‘David Armstrong and Realism about Colour.’)
Armstrong himself puts it this way:
In the manifest image of the world , to adopt Wilfrid Sellars' s expression, colour, heat and cold, sound, taste and smell, play a conspicuous part . Yet as far back as Galileo (with intellectual precedents stretching back to the Greek atomists) physicists could find no place for these qualities as intrinsic properties of physical things ...
What is a scientific realist to do? A traditional solution is to use the mind as an ontological dustbin, or sink, for the secondary qualities. Locke gave us a classical formulation. For him, the surface of a ripe Jonathan apple can properly be said to be red. But what constitutes its redness is only this. The surface has nothing but the primary qualities. In virtue of certain of these properties, however, properties of the micro-structure of the surface, it has the power to produce in the minds of normal perceivers in standard conditions (this last fills out Locke a little) ideas, or sense-impressions or sense-data, mental objects which have a certain simple quality, a quality unfamiliar to persons blind from birth ...
[But if] we have not merely accepted a scientific realism about the physical world, as Locke did, but have also made the mind part of that world, then there is no hiding place down in the mind for the sensible secondary qualities. The same reasons that made one want to exclude them from the physical world will make one want to exclude them from the mind. (Armstrong, ibid.)
If we agree that everything (including the mind) is material, then there is no longer any gain to relocating colours (sounds, smells, etc.) to the mind. At least, the early modern reason is no longer available. The question then is whether there are other reasons for relocating colours to the mind. Quite a few have been offered but the sting of attack has largely been defused and philosophers generally have no trouble warding off these other objections while counterattacking for good measure. (See below.)










Here’s how Sydney Shoemaker puts it:
It is, at any rate, one of the cliches of the history of philosophy that the problem of secondary qualities first faced in the 17th century was solved, or swept under the rug, by relegating them to the "dustbin of the mind." It is also a cliche of recent discussion of these issues that this solution becomes problematic if one abandons the dualistic view of mind taken for granted by the 17th century writers in favor of a materialist or physicalist view. A dualist can hold that there is no place for redness-as-experienced in material bodies, and nevertheless allow for its existence, by locating it in the non-material realm he takes the mind to be. A materialist obviously cannot do this, since according to him there is no non-material realm. A materialist who does not deny that there is such a thing as redness-as-experienced must allow that there is, after all, a place for this property in the material world. And if he nevertheless agrees with the 17th century writers in holding that such properties are instantiated only in the mind, i.e., as properties of sense-experiences or the like, he must explain how they can be instantiated in one part of the material world, namely the neural substrate of our perceptual experience, if they cannot be instantiated in the parts of the material world in which we seem to experience them as being instantiated, namely the surfaces of objects like ripe tomatoes ... What options does a materialist have? One is to say that the 17th century writers got the whole discussion off on the wrong foot, and that there is no cogent objection to a straightforward identification of the secondary qualities with "primary" ones, i.e., with properties that have a place in the scientific world picture ... (Shoemaker, ibid.)
Shoemaker himself does not think that colours are out there in the world, but proposes a more sophisticated view which need not concern us here.






Second, that philosophers sometimes use terms like ‘colour,’ ‘sound,’ ‘smell’ (etc.) in an ambiguous way which is apt to mislead those who are not mindful of the ambiguity. Consider the old riddle, doubtless inspired by the Galilean view of secondary qualities: if a tree falls in a forest and no one is around, does it make a sound? A straightforward Galilean answer would be that it doesn’t, since sound is only ever a “sensation” in the mind. A more nuanced answer however is that, in the sense in which sound is a “sensation” or a “mental experience,” the tree doesn’t make a sound, whereas, in the sense in which sound is a physical disturbance in the air, it certainly does. By these nuanced lights, while the term ‘sound’ perhaps has one sense in which it denotes something “in the mind,” it has another legitimate sense in which it straightforwardly denotes something in the external world. Given this distinction, it is possible for a philosopher (say) to claim that secondary qualities like colour, sound and smell are located in the external world even as he or she holds firmly to the broad Galilean view outlined above. This philosopher would simply have to be using terms
like ‘colour,’ ‘sound’ and ‘smell’ in the second of the two senses mentioned. This is a potential source of confusion to someone not in the know but, as we shall see, a position like this is in fact often attributed to Locke. It gets even more confusing when 20th-century philosophers enter the picture because many of them operate with an unspoken theoretical model on which the second of the two senses above is the “correct” way to use terms like ‘colour,’ ‘sound’ and ‘smell,’ so that there can be no question as to whether colour, sound and smell are out there in the external world – of course they are. It is not only outsiders who stand to be confused by these linguistic twists and turns; philosophers themselves often confuse each other.