Wednesday, July 23, 2014

Colors and their Objective Basis
In 1976, when I delivered the John Locke Lectures at Oxford, I often spent time with Peter Strawson, and one day at lunch he made a remark I have never been able to forget. He said, "Surely half the pleasure of life is sardonic comment on the passing show".  This blog is devoted to comments, not all of them sardonic, on the passing philosophical show.
Hilary Putnam


I will begin by discussing a question from my color-skeptical friend, who recently sent me a beautiful color-illusion. I have been unable to find it online, but the illusion involves a picture crossed by a horizontal white line. From this line, in the center of the picture, a vertical line white line runs straight down. The upper left quadrant of the picture is solid red (the right edge of the red square is just above the vertical white line), and the upper right quadrant is solid green. In addition, there are two white dots or small white circles, a slightly larger dot in the upper half of the picture, above the vertical white line, and a smaller white dot in the lower half of the picture, right on the vertical white line. The bottom half of the picture (the half below the horizontal white line) shows a landscape with  brown hills below a blue sky with clouds. On a second look, one sees that the right side of the bottom half is the mirror image of the left half, i.e., the lower left quadrant is “reflected” around the vertical white line.
The instructions that go with the illusion are as follows: “fix your gaze on the upper white dot and count to 50. The fix your gaze on the lower white dot.” When I tried it, the effect was startling. When I shifted my gaze from the upper to the lower white dot, the hills in the lower right quadrant appeared to change from brown to a verdant green! (This is called a “color accommodation response”.) The look of the lower left quadrant was unaffected, so that the two halves of the picture below the horizontal white line were no longer mirror images—not as far as the colors I seemed to see were concerned!
My friend wonders how “color can be objective in the face of such variation”.
I shall begin with my answer to his question, and continue in future posts with a further discussion of “looks”.

Colors and color-looks
If someone had been standing next to me while I carried out the above instructions, they would not have observed any change in the picture. And indeed the colors on the screen (or on the page, if the illusion had been printed in a book) would not have changed. The “variation” my friend speaks of is not a variation in the actual colors. It is a variation in the look of certain colored regions, not in the actual color of those regions. Similarly, when I turn out the lights in my bedroom at night, my slippers (which I can barely see in the dark) no longer look light gray but almost black. And when I put the milk bottle in the refrigerator, it no longer looks white. Unless the refrigerator light fails to go off and there are little people in the refrigerator to look at it, it doesn’t “look” any way at all. But my slippers are light gray, even when one can’t make that out because the room is too dark, and the milk bottle is still white, even when one can’t see it. By assuming that colors are looks, my friend begged the question for the antirealist side.
But didn’t I beg it as well by assuming there is such a thing as the “actual color”? That is the question to which I now turn.

The objective basis of color discriminations
One obvious fact about colors is that some things are indistinguishable in color. Specifically, a uniformly colored part of a surface, for example part of a wall or of the surface of an object, may “match” a color sample. People with normal color vision will not be able to distinguish any difference in the color of the surface or surface-part and the color of the color sample, say a colored square on a chart provided by a paint company or an interior decorator. And this “matching” relation will persist under the same viewing conditions and for normal viewers, unless the surface or surface-part in question undergoes a physical change.  The obvious, indeed trivial, explanation is that whatever the “nature” of color may be, colors have an objective basis. They are not just “in our heads”. Whether the “objective basis” is the color (the color physicalist position) or is only the explanation of the intersubjectivity of color-judgments is a further question. But that there is an objective basis is, I believe, undeniable. Otherwise, how are we to explain that the traffic lights change their colors as they are supposed to, and as a result of a physical  signal?
By using the word “objective” here, I do not mean only that the properties that cause surfaces or parts of surfaces to match color samples are properties about whose possession or non-possession there is a fact of the matter  (metaphysical objectivity), but they they are viewer-independent, in fact physical, properties of the objects.  They are not fundamental physical properties, or simple functions of the fundamental physical properties, nor are they of interest to physicists who are seeking fundamental laws. But we have a reasonable account of them: they are reflectance types. Here I agree with Alex Byrne and David R. Hilbert, who write (“Colors and Reflectances,” in Readings on Color, Volume 1: The Philosophy of Color, MIT Press, 1997; online at
But is there a physical property that all and only (actual and possible) green objects share? Given our assumption about the general correctness of our color perceptions, the answer is (plausibly) yes. The property that all green objects share is a type of SSR [=Surface Spectral Reflectance - HP]. Very roughly, this property -- call it ‘SSRGREEN’ -- is the type of SSR that allows an object in normal illumination to reflect significantly more light in the middle-wavelength part of the spectrum than in the long-wavelength part, and approximately the same amount of light in the short-wavelength part as in the rest. Obviously, particular reflectances meeting these specifications -- for instance those of frogs, lettuce, and dollar bills -- may be otherwise very different.
The property green, if it is this type of SSR, is not a particularly interesting property from a physical point of view. Since we only find it salient because our perceptual apparatus is built to detect it, it might be called an anthropocentric property (cf. Hilbert 1987). Alien physicists lacking our visual apparatus would not need to single it out for special attention, unlike the property of having charge e, or spin 1/2. (These aliens might likewise find the visible spectrum no more than an arbitrary segment of the entire electromagnetic spectrum.) But that does not at all impugn the status of an idiosyncratic type of SSR as a physical property that is ‘objective,’ in almost every sense of that protean word. Particular SSRs are not in any philosophically interesting sense dependent on human beings; neither is the type, ‘either SSRa or SSRb or SSRg, ... ,’’ where these particular SSRs seem from a physical standpoint to be a motley collection.”
I would add, moreover, that the property green, and many other color properties, can be discriminated by several non-human species as well, including (after appropriate training) domestic cats.
Why we should identify the SSR-types of which Byrne and Hilbert speak with colors
Sometimes the basis for calling something an X does not support the appellation X. For example, if the basis for calling someone a witch is that her community believes that she causes other people to suffer illnesses and other misfortunes by means of black magic, then the appellation wrong and the basis is insufficient.  If the basis for calling a surface green is that we mistakenly believe it have a property that nothing could possess, greenness, then even if the objects we call green have a common property that prompts our brains to classify it as “green”, the basis is insufficient. But the only serious arguments I have seen to support the idea that nothing “out there” could actually be green are: (1) Russell’s argument in Problems of Philosophy that “the colors we see” depend on the viewer, an argument that conflates colors and looks of colors; and (2) Sellars’ argument that physical properties aren’t what color, solidity, and the like, are “presented as”, an argument that assumes we cannot refer to a property by a term if our mental image  of the property does not resemble the property described by science—an argument that is an obvious descendant of the 17th century empiricist ideas that our “ideas” are mental images and they refer to what is similar to them. In my previous post I argued that these arguments lead to skepticism not just about colors but about all the properties of objects, and that they depend on untenable requirement on what it takes to refer to something real. If we reject them, as we should, I see no serious reason not to agree with Byrne and Hilbert’s “physicalism” about color.
I know that more needs to be said about “looks”. And I will say more – in future posts.














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