There is, however, a radical difference between the two systems. The standpoint of the Aristotelian classification is the predication of one universal concerning another.
This extreme of nominalism for which predication is impossible is, however, compromised by two concessions.
Yet from the points of view alike of an absolute pluralism, of a flux, and of a formula of bare identity - and a fortiori with any blending of these principles sufficiently within the bounds of plausibility to find an exponent - all knowledge, because all predication of unity, in difference, must be held to be impossible.
The Porphyrian, by introducing species, deals with the predication of universals concerning individuals (for species is necessarily predicated of the individual), and thus created difficulties from which the Aristotelian is free (see below).
The first difference then between the Categories and the Metaphysics is in the nature of an attribute; and the theory of inherence in the Categories is nearer to Plato and more rudimentary than the theory of predication in the Metaphysics.
A nominalist to the core, he held that definition and predication are either false or tautological.
Again, when it presently appeared that the theory of the immanent idea was inconsistent with itself, and moreover inapplicable to explain predication except where the subject was a sensible thing, so that reconstruction became necessary, the Zenonian difficulty continued to demand and to receive Plato's best attention.
On the other hand, in the Metaphysics, the distinction between inherence and predication disappears; and what is more, the relation of an attribute to a substance is regarded as so close that an attribute is merely the substance modified.
Now, when without qualification he affirmed that the unlike thing cannot be like, nor the like thing unlike, he was on the high road to the doctrine maintained three-quarters of a century later by the Cynics, that no predication which is not identical is legitimate.