The Metaphysics of Aristotle

Here genus is signified by one idea extended through many: for genus is not an aggregate of species, as a whole of parts, but it is present to every species, to which it is at the same time prior.

But every species subsisting separate from other species, and from genus itself, participates of genus. By many ideas, different from each other, but externally comprehended under one idea, which is genus, species are signified; externally comprehended indeed, as being exempt from species, but comprehending the causes of species: for genera, truly so called, are both more ancient and more essential than the species which are ranked under them. Of genera, also, some have a subsistence prior to species, but others subsist in them according to participation. To perceive these two, therefore, viz. one idea extended through many, the individuals of which subsist apart from each other, is the province of the divisive power of dialectic; but the other two pertain to the definitive power of this art: for definition perceives one idea through many wholes conjoined in one, and collects into one definite conception many ideas, each subsisting as a whole. It also connects them with each other, and perfects one idea from the assumption of all wholes; conjoining the many in one. Besides this, it considers the many which it has collected in one, lying apart, and the whole which is produced from them.

Plato has given most happy specimens of that part of the dialectic art...