Colors and Powers Again
Locke distinguishes primary qualities from powers to produce sense impressions of them in perceivers, but he thinks that secondary qualities are “nothing else, but several powers in them, depending on these primary qualities…to produce several different ideas in us”.  That is, he identifies colors with powers to produce states of mind: that is what a color is—a causal power of a certain sort. I am going to make two rather brutal criticisms of this doctrine. The first is that causal powers are invisible but colors are not. Suppose I see an object as square: that quality is visible to me, but the power to produce an impression of square is not so visible. I don’t see an object ashaving such a power—I simply see it as having the geometric property of being square. As Hume taught us, we have no sense impressions of causal powers—though objects have them. Powers are actually rather mysterious things involving potentiality, and they concern relations between objects and other objects (in this case objects and perceiving minds). They aren’t intrinsic manifest qualities; they are rather like modal properties such as being possibly square. They aren’t things the senses can resonate to. So Locke’s theory implies that colors are invisible! Nowhere does he acknowledge that consequence, and it is indeed startling: surely we want a theory of color to be consistent with the visibility of color. I suppose he could just accept the consequence, but it is a hard bullet to bite. Maybe colors have the power to produce impressions of themselves, as shapes do, but they are not identical to such powers. In fact, the power to produce sense impressions depends on certain forces that exist in objects, but forces are not perceptible: we don’t perceive electrical or gravitational forces, only their effects. Yet we see colors quite plainly: they are nothing like hidden powers or potentialities.
Second, external physical objects are not the only things with such powers. Minds and brains have them too. Your mind has the power to produce ideas of secondary qualities in you, as it does when you hallucinate. In fact, it must have such powers if external objects are to elicit sense experiences in you; the external object alone cannot do this. The brain too must have the power to generate sense impressions, which it demonstrates all the time. But if colors are identical with such powers, then minds and brains are colored. They have these powers in virtue of possessing appropriate primary qualities, just like external objects, so they have the property Locke identifies with color: but they don’t have the very color that this power would entail. The mind isn’t red when it exercises the power to produce impressions of red, and neither is the brain. It is easy to see what is going wrong here: having the power to produce sense impressions is just too broad a condition to capture what color is. Couldn’t a super-scientist have such a power without having the colors that allegedly go with it? Nor will it help to limit the power to the surfaces of external objects, since they too could have such powers and not be colored: they might be little minds or brains. Having the power to elicit experiences of red will never add up to being red—merely to the ability to cause experiences thereof (almost anything can do that in the right circumstances). The condition is clearly far too weak.
So the Lockean theory of color renders color (a) invisible and (b) a property of the mind-brain. As I say, brutal.