Color and Causality
Color and causality don’t mix: causality doesn’t mention color and color is indifferent to causality. Shape is very different: shape always affects causal powers. Shape and causality are made for each other, while color and causality are complete strangers. This means that ordinary objects have two aspects—causal and non-causal. We know this from everyday perception: we see the way shape affects causality as objects interact, but we never see colors affecting objects causally. Objects don’t attract or repel each other in virtue of color; color doesn’t affect the laws of motion; objects don’t change the color of other objects in virtue of their color. This is why physics has no time for color; it simply isn’t a causal variable. Perhaps we can imagine a world in which color does exercise causal powers: objects move differently according to color; they have attractive and repulsive forces as a function of color; the color of one object can change the color of another if it gets close to that object. But this strikes us, rightly, as unintelligible: how could color exercise such causal influence (compare moral properties)? In any case, it isn’t so in our world; nor does this seem like an accidental fact about color. Color is epiphenomenal. This fuels projective views of color: the reason colors lack causal powers is that they are projected by the mind onto objects; they are not intrinsic to objects. Perceived color is a kind of illusion: colors don’t exist in objects but in the mind, and a distant mind can’t affect the causal powers of an object. If you experience a visual illusion, the properties attributed in the illusion won’t affect the causal powers of the object (the apparently bent stick in water won’t behave like a bent stick in its causal interactions). Nor does seeing-as affect the causal powers of an object according to the aspect seen. Color perception is like that, according to projectivism, so of course color is epiphenomenal: it isn’t even a constituent of external reality. But even a realist about color will concede that colors have no causal powers: they exist in objects but they make no causal difference to how objects behave. Figure, mass, and force make a causal contribution, but color is causally idle.
So much is generally (though not universally) accepted, if not always with complete equanimity. Why are colors there if they make no causal impact? Are they really in objects at all? The point I want to make, however, has not, to my knowledge, been stated: namely that colors so understood make trouble for the causal theory of perception (or maybe the causal theory of perception makes trouble for colors). That theory says that perception requires three conditions to be met: (i) the object has the property, (ii) the perceiver has the impression of the object having the property, and (iii) the object’s having the property causes the impression that it does.  There are refinements that need to be made, but it is generally accepted that the causal condition is at least a necessarycondition for veridical perception to occur. But colors have no causal powers, so how can they be seen? Colors can’t cause sense impressions in perceivers because they can’t cause anything: the color red, say, doesn’t cause sensations of red in perceivers by interacting with the rods and cones, since it has no causal powers. It is true that the surface of the object reflects light waves that cause disturbances in the receptor cells of the retina, but that is not what the color is—or else colors would have causal powers and be studied by physics. We can of course stipulate that “color” is to refer to such physical phenomena, but these phenomena are not colors-as-we-see-them—those things are causally inert. So colors in the ordinary sense cannot be the cause of impressions of color–yet we see them all the time. On the face of it, then, the causal theory of perception is false for colors, though not for shapes: you can see a property that does not cause you to see it. Colors are visual objects par excellence but they are not causes of visual sense impressions. If it were possible to see an object only as colored, then we would have the result that an object can be seen and yet have no causal contact with the perceiver. Or to shift to another type of secondary quality: if a piece of sodium chloride could be tasted only as salty, with no other quality perceived, then perception of an external physical object could occur in the complete absence of any causal contact with its manifest properties. For secondary qualities in general are causally impotent, and ex hypothesi they are the only ones being perceived in such a case. It would be logically possible to perceive all the objects of the external world and yet have no causal contact with those objects.
Of course that is not the only possible response: someone might maintain that colors are not perceived precisely because they have no power to be perceived, as the causal theory implies. Instead they are hallucinated as a result of photon bombardments; they don’t really exist in objects at all. There is thus no veridical perception of color. If this is felt to be ad hoc and implausible, we could try a third approach: colors are invisible. They exist in objects and we have impressions of them, but they are not seen at all: what we have here is a case of veridical hallucination. The object is red and it looks red, but because of the lack of causal connection it can’t be perceived—so it’s literally invisible. I don’t know of anyone who has adopted this view, and one can understand why: to say that colors exist in objects, and that we have impressions of them, but we don’t really see them, is radically counter to common sense. We would need some motivation for this combination of claims that goes beyond simply observing that the perception of color is incompatible with the causal theory of perception. For the natural conclusion is that color perception refutes the causal theory of perception, since we can see properties that don’t cause us to see them. Or is it that they have a magic power to whisk causation into existence when and only when a visual system passes by? My own conclusion is that color perception is a straight counterexample to the causal theory, even as a theory of the necessary conditions of perception (there are well-known problems about its sufficiency). The intuitive idea behind the theory is simply wrong; the theory is at best partially true. The simple fact is that you can see the colors of objects without their causing you to do so. 
 One might try to weaken the theory so as to require only that a suitably connected property be the cause of the sense impression not the color property itself—as it might be, the light waves correlated with the color. However, this runs into immediate problems such as fact that the seen color will not be caused by the color itself but by a distinct property, so why is the impression not a misperception of that property? And surely we don’t want to end up saying that the only true causes of sense impressions are basic physical properties, with the consequence that only such properties are really perceived.