In philosophy, as in life, asking the right question is usually a substantial part of finding the answer. This was beautifully demonstrated this semester with two public lectures organized by the philosophy department. Both lecturers were from outside Bowdoin, dealt with color, and drew full halls, but that is where the parallel ended. The first speaker, Larry Harding from Syracuse University, started with a description of light, a matter of physics, where the question "what is light?" can be answered objectively. He then progressed to aspects of the physiology of human vision, and finally arrived at the partially subjective question of color, because it combines light and visual perception. It was a very stimulating talk.
The speaker this week, Alex Byrne of MIT, instead asked the wrong question: "What is color?" Now "Is snow white?" may be a catching title, but it is also a futile (because unanswerable) question, since snow merely reflects the light of the visible spectrum that it receives. Consequently, snow is experienced as white when seen in white light, yellow-orange when seen with the light of a sodium lamp, and colorless in the absence of light. No amount of philosophizing can change that. With a lemon, matters are a little more complicated, because its surface selectively absorbs some colors more than others, and hence it adds its own color filter to light reflecting off it, as can be described objectively in terms of its reflectance spectrum.
Indeed, as long as we leave out occasional nonlinear complications such as fluorescence, the spectrum of the light reflecting off a non-transparent object can be described accurately by the convolution of the spectrum of the incoming light and the reflectance spectrum of its surface. We know this because of the highly realistic quality of digital simulations using this method in, say, architectural drawings and sophisticated computer games, in which case the partially subjective parts of the chain, those "in the eye of the beholder," are of course kept constant.
Likewise, when we consider the cones and the rest of the brain as non-interacting entities, the perceived color of an object is the convolution of one, the spectrum and directionality of the incoming light, two, the reflection spectrum of the object plus directional changes resulting from its surface texture, three, the spectral response of the rods on the retina, and four, the spectral description of color processing in the neurons on the retina and in the rest of the brain. The word convolution is used here in its strictly mathematical sense. Since the spectral sensitivity of the rods is known to show slight variations from one human to another, it is hardly surprising that there is no such thing as, for example, a "unique green." And the perception of color in the brain is, certainly, a highly individualized process, further complicated by the fact that the human brain adds its own correction for the perceived color of the ambient light. Consequently, the question of the "intrinsic" color of such objects has no completely objective answer, as the second lecture (though not its lecturer) made abundantly clear.
The juxtaposition of these two talks on the same topic was certainly quite enlightening.
Robert de Levie is a Visiting Scholar in Chemistry at Bowdoin.