It is often asked whether or not a person blind from birth can form a concept of a color such as red. The conventional response is negative, since such a person obviously cannot imagine what a color "looks like." Yet the blind have indirect experience of the world in the form of communications from others, and (like all humans) must organize that experience into concepts in order to make sense of it. A blind person will, for example, hear multiple observers refer independently to the same objects as being "red"—an experience which can be interpreted logically only by understanding those objects as differing from other real-world objects in some way. Therefore that person can and must form a concept red in order to integrate his or her total experience of the world.

As an analogy, a scientist might form the concept quark based on indirect cloud-chamber evidence, even though no scientist has seen a quark directly. So the concept red has the same meaning to a blind person as to any other—that is, it is an attribute of all red objects. In the blind person's personal hierarchy of concepts, however, red appears at a relatively abstract level, just as quark is an abstract concept in a physicist's knowledge.