A criticism of forms relates to the origin of concepts without the benefit of sense-perception. For example, to think of redness in general, according to Plato, is to think of the form of redness. Critics, however, question how one can have the concept of a form existing in a special realm of the universe, apart from space and time, since such a concept cannot come from sense-perception. Although one can see an apple and its redness, the critic argues, those things merely participate in, or are copies of, the forms. Thus, they claim, to conceive of a particular apple and its redness is not to conceive of "applehood" or "redness-in-general", so they question the source of the concept.