Plato: Forms as genera and species

Successful development of the theory of forms depended upon the development of a distinction between two kinds of predication. Plato held that a sentence making a predication about a sensible particular, “A is B,” must be understood as stating that the particular in question, A, displays a certain property, B. There are ordinary predications about the forms, which also state that the forms in question display properties. Crucially, however, there is also a special kind of predication that can be used to express a form’s nature. Since Plato envisaged that these natures could be given in terms of genus-species trees, a special predication about a form, “A is B,” is true if B appears above A in its correct tree as a differentia or genus. Equivalently, “A is B” has the force that being a B is (part of) what it is to be an A. This special predication is closely approximated in modern classifications of animals and plants according to a biological taxonomy. “The wolf is a canis,” for example, states that “wolf” appears below “canis” in a genus-species classification of the animals, or equivalently that being a canis is part of what it is to be a wolf (Canis lupus).

Plato’s distinction can be illustrated by examples such as the following. The ordinary predication “Socrates is just” is true, because the individual in question displays the property of being just. Understood as a special predication, however, the assertion is false, because it is false that being just is part of what it is to be Socrates (there is no such thing as what it is to be Socrates). “Man is a vertebrate,” understood as an ordinary predication, is false, since the form Man does not have a backbone. But when treated as a special predication it is true, since part of what it is to be a human is to be a vertebrate. Self-predication sentences are now revealed as trivial but true: “the Beautiful is beautiful” asserts only that being beautiful is (part of) what it is to be beautiful. In general one must be careful not to assume that Plato’s self-predication sentences involve ordinary predication, which would in many cases involve problematic self-exemplification issues.

Plato was interested in special predication as a vehicle for providing the real definitions that he had been seeking in earlier dialogues. When one knows in this way what Justice itself really is, one can appreciate its relation to other entities of the same kind, including how it differs from the other virtues, such as Bravery, and whether it is really the whole of Virtue or only a part of it.

By means of special predication it is possible to provide an account of each fundamental nature. Such accounts, moreover, provide a way of understanding the “pure being” of the forms: it consists of the fact that there cannot be a true special predication of the form “A is both B and not-B.” In other words, special predication sentences do not exhibit the phenomenon of rolling around between being and not being. This is because it must be the case that either B appears above A in a correct genus-species classification or it does not. Moreover, since forms do not function by being exemplars of themselves only, there is nothing to prevent their having other properties, such as being and unity, as appropriate. As Plato expresses it, all forms must participate in Being and Unity.

Because the special predications serve to give (in whole or in part) the real definitions that Socrates had been searching for, this interpretation of the forms connects Plato’s most technical dialogues to the literary masterpieces and to the earlier Socratic dialogues. The technical works stress and develop the idea (which is hinted at in the early Euthyphro) that forms should be understood in terms of a genus-species classification. They develop a schema that, with modifications of course, went on to be productive in the work of Aristotle and many later researchers. In this way, Plato’s late theory of the forms grows out of the program of his teacher and leads forward to the research of his students and well beyond.