Color and a priori knowledge

Some truths about color are knowable a priori. For example, it is knowable a priori that redness is not identical to the property of being square. This extremely modest and plausible claim has significant philosophical implications, or so I shall argue. First, I show that this claim entails the falsity of standard forms of color functionalism, the view that our color concepts are functional concepts that pick out their referents by way of functional descriptions that make reference to the subjective responses of perceivers. I then argue that, while some sophisticated forms of color functionalism can accommodate the a priori knowability of a truth like "redness is not identical to squareness," they can only do so by abandoning color realism, the thesis that colors are instantiated by external material objects. In practice, color functionalists are almost invariably color realists. Thus, given extremely modest assumptions concerning what can be known a priori about color, we should conclude that color functionalism, at least in its typical realist form, is false.