The term response-dependent was introduced by Mark Johnston (1989) for concepts, such as red, that support an a priori biconditional on roughly the following lines: "X is red if, and only if, X is such that it would look red under normal conditions." Any concept of the intended kind will apply to something just in case the object has a property in virtue of which it would elicit a relevant response, on a par with the appearance of redness, under relevantly favorable conditions; it will be akin to the concept of a secondary quality, traditionally conceived. The response to be elicited will involve a cognitive impression, so that the object looks, seems, or presents itself in a certain manner. And the conditions under which that response is guaranteed will have to be capable of independent specification; they cannot be defined just as whatever conditions will provide the...