A principle doubtfully attributed to Plato on the basis of passages in the Republic and Parmenides. If we count many leaves, then in addition to the leaves we count there is one thing—a leaf—which they all are. This thing is known to us separately before we can recognize or count the individual leaves. The principle poses one version of the problem of universals. It does not seem easy to reconcile with Plato's general position that he is more sure about the existence of some forms (e.g. for terms with opposites) than others, since if forms are necessary for any kind of classification, we should be happy with all of them. See also third man argument.