Color and Perception
For color, to be is to be perceived: not so for shape. That was a central tenet of the modern philosophers. Color depends on perceivers for its existence, but shape does not. Thus it makes no sense to suppose that colors exist in a possible world and yet no one perceives them—not humans and animals, not God, and not the colored objects themselves. For what colors would they be? There is nothing to determine what color an object is except how it is perceived (in normal conditions by normal perceivers). The same is not true of shape: shape is determined by how the object behaves in relation to other objects, i.e. by its causal powers. For shape, to be is to be causal. There is something other than perception to fix the shapes of objects. But there is nothing other than perception to fix the colors of objects–hence the vacuity of the idea of a colored world devoid of any perceivers whatsoever. We can certainly subtract a sub-class of color perceivers from a world and leave color intact, but we can’t take allperceivers away and expect to be left with color in all its glory. Suppose colored objects all had eyes and could see their own color: we could remove human and animal perceivers, and even any divine perceivers there might happen to be, leaving only the sighted objects themselves; but we had better not remove the eyes of these remaining perceivers if we want to leave color intact. Nothing like this is true of shape, however, since the causal profile of objects can survive their not being perceived (it is “objective”). This is why people call shape a “physical” property and color a “mental” property: it is a matter of perceiver-dependence, or the lack of it. Most modern philosophers thought the necessary perceivers were human (or possibly animal); Berkeley thought they were human and divine; someone else might suppose that the objects themselves are the perceiving subjects of their sensory qualities. The common thread is that colors need perceivers, because their esse is percipi. Shapes, on the other hand, don’t need perceivers, because their esse is not percipi, but rather causal or mechanical or physical. This is the crux of the distinction between primary and secondary qualities, as it has been traditionally understood. Secondary qualities are the ones that are inextricably linked to mental acts of perceiving in such a way that perception is both necessary and sufficient for their existence (by normal perceivers in normal conditions, etc.). Without suitable perceivers, then, color has no ground of being. It is the same for other secondary qualities, perhaps more obviously so: no world has tastes and smells in it that lacks all perceiving subjects—human, animal, divine, or objectual. For what tastes and smells might these be? Tastes and smells don’t affect inter-object causal powers, so they have no ground beyond perception: to have a certain taste is to taste a certain way to perceivers—not to propel objects in a certain direction that interact with the tasty object.
Does this imply that secondary qualities are mental (subjective, psychological)? Not strictly, since they may be non-mental qualities that are fixed by mental facts: they might be a special type of primitive property that depends for its instantiation upon mental facts about perceivers. But (a) that would be a pedantic distinction in the absence of some principled reason for withholding the term “mental” from them; and (b) even granting the point we still have the conclusion that colors require mental acts of perceiving, which is all the metaphysician of color is really interested in (it will be sufficient for object mentalism certainly). True, we don’t normally call colors “mental” or “psychological” for boring conversational reasons (the implicatures are uninviting), but from a theoretical point of view there is nothing amiss with extending the use of the term in this way—since colors do depend on mental acts strictly so called. The important point is that colors (etc.) contrast with shapes (etc.) in their relations to perceivers, with the former presupposing them and the latter quite indifferent to them. We can simplify by saying that secondary qualities are mental and primary qualities are not mental. Then we can derive appropriate conclusions from this distinction, notably that colors need perceiving subjects and shapes don’t. Of course, if we were to claim that colors are reducible to wavelengths, then the distinction would collapse and colors would not depend on perceivers ; but I am assuming here that this is wrong and that the choice is between a sui generisclass of non-mental properties and inclusion in the class of mental properties. And the point I am making is that for colors and other secondary qualities, to be is to be perceived by some perceiver or other: take away all perceivers and you take away color. Or better: if there had never been any perceivers, there would be no colors (tastes, smells, etc.)—just as, if there had never been any organisms, there would be no poisons or fearsome objects or healthy life-styles. And which colors an object has depends on how it is perceived: there is no experience-transcendent criterion of color distribution. The same is not true of primary qualities, since they play a role in the causal workings of the world as described by physics.
We can imagine someone jibbing at the idea that pains are aptly described as mental on the grounds that pains are located in parts of the body and hence are “physical” (“mental” pain is more like inner anguish). The same might be said of perceptual sensations, which are located in or near the sense organs. This fastidious thinker reserves the word “mental” for thoughts, which seem to occur in the inner sanctum of the self. One can appreciate the motivation for this policy of verbal purity, but it doesn’t affect the point at issue: even if we accept it, we can still assert that pains and sense perceptions require a perceiver in order to exist, along with a mental act of apprehension. So they are “mental” by association. They are not like states of the body that require no apprehending subject in order to exist. Even if pains and sense perceptions are not rightly described as mental, they still presuppose a perceiving subject, and that is the central point at issue. Likewise for colors, tastes, etc.: even if they are declared sui generis—neither mental nor physical—we still have the conclusion that they are dependent on a psychological subject. Supervenient facts don’t have to be of the same type as the facts on which they supervene, but they still presuppose those facts—they can’t exist without them. From the point of view of object mentalism, then, it is all one whether we call colors mental or assign them to a separate category; all that matters is that they are perception-dependent. The question at issue is the identity of the relevant perceivers—whether human, animal, divine, or the objects themselves.
The point on which Berkeley was particularly insistent is that mental qualities cannot inhere in a purely material substance, as many had supposed. If color is mental (or dependent on the mental), then it needs a distinctively psychological subject: it can’t just be the same material substance that primary qualities happily inhere in. So when an object is both cubical and red it needs two supporting substances, one for its primary qualities and one for its secondary qualities (a self in short). If we retain the ideas of primary qualities and material substance (contra Berkeley), then we need to add a suitable substance for the secondary qualities, construed as mental, viz. a perceiving subject. The question then is what or who this subject is—human and animal minds, the divine mind, some sort of immanent cosmic mind, or the mind inherent in the object that has the quality in question. But there should not be much dispute about the conception of secondary qualities that powers this debate, i.e. their essential connection to perception. 
 For someone who holds that the world could contain all the secondary qualities it now contains in the complete absence of anyone to perceive them, the obvious question is what might confer these qualities on the world. Surely these qualities reflect the survival needs of organisms and are not simply found to exist in the world ab initio. Could it be the case that all organisms misperceive the real colors and tastes of things, these being determined quite independently of how the world seems? Of course, the physical basis of such qualities could exist in the absence of perceivers, but could colors-as-we-perceive-them so exist? Is the redness of red something that exists independently of the appearance of red to perceivers? What if one group of perceivers sees tomatoes as red and another sees them as green—is one right and the other wrong (or both are wrong)?