Let us begin by trying to form a purely physical conception of the world. I think there is little doubt that, in the past, that is the idea from which the unreality or the "subjectivity" of color has derived its greatest support. The thought is that the world as it is independently of us is fundamentally a physical world, a world of purely physical goings-on. That was the basic idea of Democritus. He started with a conception of a world of nothing but atoms moving in a void, and everything else we believe in was therefore said to have only a human or "conventional" source. Galileo shared that same general idea of the physical world, but what added enormously to its richness and power in the sixteenth and seventeenth centuries was the new idea of a mathematical science which ideally would explain why everything happens in the physical world as it does. That is what posed the question of the metaphysical status of color in its most challenging form. How could color (or anything else that is not included in that austere physical conception) be fitted into a world that in itself is purely physical? Whatever is not capturable in that basic science must be at best dependent on perceiving animals and would be nothing at all if they were taken away. Color then seemed inevitably to find a place in the world only as something "subjective," as dependent in some way on perceivers and their experiences.

Descartes endorsed and advanced the idea of a purely physical science but without believing that the world is exclusively physical. The world for him contained not one but two distinct kinds of things. One of them was body, whose essence was extension, or taking up some space. Every property a body could have therefore had to be a "mode" of extension--a way of being extended in space. And color was said to be no such "mode" of physical things. It was not referred to by any of the terms needed for a clear and distinct conception of body. It could be understood only as something dependent on happenings in the other world of purely mental things, whose sole essence it is to think. Color could be put together with a purely physical world and the physical world could be put together.

J. L. Mackie in our own day regards our ascription of colors to things as "a mistake, a systematic error." That is because he thinks that when we make such ascriptions, "all that is out there in reality" are certain material bodies which in themselves have only such qualities as shape, size, position, number, motion-or-rest, and solidity. Color is excluded from objective reality because the world is exclusively physical, and color has no place in that purely physical world.

When J. J. C. Smart says that colors are not part of the world as it is in itself, he means that they "are not part of the world as described in the physical sciences." "The scientific description of the world," he says, "makes no use of words for color." What it is in an object that gives rise to a perception of color is "the state of its minute parts," and he means the physical state of those parts. "The physicist does not need the word 'red': he can do what he wants simply by talking about wavelengths of light." And the world as described by the physician (if what he says is true) is the world as it is in itself, non-"anthropocentrically" described.

Bernard Williams above all stresses the essential connection between the idea of "the material world as it is understood by natural science" and the idea of the world "as it really is." The scientific conception is arrived at by "correcting for" the peculiarities of various observers, so it ideally bears no signs of whose conception it is. That is, in part, why the scientific conception above all others can be taken to represent the world as it really is independently of all observers; it is not just how the world appears to us. Williams thinks colors will be left out of any "absolute" conception of an independent world because the natural science of the material world is "a conception in which color does not figure at all as a quality of the things" in that world.

Common to all these views of the world as it is independently of us is the idea that, in Mackie's phrase, "all that is out there in reality" are certain physical things interacting in accordance with certain physical laws. That is, in Smart's phrase, "the way things are in nature." For Williams, it is the way the world "really" or "fundamentally" is.

Physical science tries to discover the laws of the world and so explain everything that happens. It is a mathematical science of the quantifiable dimensions of bodies, motions, and forces. It speaks of light and its transmission from one region to another, for example, but not of the colors of any things in those regions. That is not to say that physical science cannot express differences among things which we think differ in color. A ripe tomato and a ripe lemon are different. But the descriptions physical science gives of those differently colored things are in terms of measurable physical magnitudes. The differences are physically described. And if "all that is out there in reality" or "in nature" are the physical things with their physical properties, those are the only differences there can be in reality between the tomato and the lemon. We humans naturally speak and think of the one as red and the other as yellow, but the features which physical science describes in those objects are something else, not colors. They are said to be the "real" differences between those objects--the only differences there are that are fully independent of all perceivers and their responses.

There is no question that the metaphysical idea of the unreality or "subjectivity" of color is given great impetus by this compelling picture of a physical world. . . . The basic idea is that the physical world contains no color.

--Barry Stroud, The Quest for Reality: Subjectivism and the Metaphysics of Color, pp. 46-48