But we get here a clue to Plato's doctrine, the doctrine of the methexis, to use his own term. He would seem to teach that whatever particular thing exists, it does so by the methexis, or participation of the idea. The idea is that which makes the thing what it is, _causa essentialis_. Thus, a man is man by participation of the man-idea, or the ideal man, humanity; a horse is a horse by participation of the horse-idea, or ideal horse; a cow is a cow by participation of the cow-idea, ideal cow, or _bovisty_; and so of a sheep, a weazel, an eagle, a heron, a robin, a swallow, a wren, an oak, a pine, a juniper. To know any particular thing is to know its idea or ideal, and to know its idea or ideal is to have true science, for it is science of that in the thing which is real, stable, invariable, and permanent. This doctrine is very true when by ideas we understand genera and species, but not, as we have already seen, and as both Rosceline and Abelard prove, when we take as ideas the abstract qualities of things. Man is man by participation of humanity; but is a thing white by participation of whiteness, round by participation of roundness, hard by participation of hardness, beautiful by participation of beauty, or just by participation of justice, wise by participation of wisdom? What is whiteness, roundness, hardness, beauty, justice, or wisdom in the abstract, or abstracted from their respective concretes? Mere conceptions, as said Abelard, or, rather, empty words, as said Rosceline. When Plato calls these ideas, and calls them real, he confounds ideas with genera and species, and asserts what is manifestly untenable.