43. The Common Essence and the Individualized Essence. -- The individual being, which is the only real substance, is alone capable of existing as such; the universal is not a thing in itself; but it is immanent in the individuals, reproduced in all the representatives of a class; it gets its independence, its isolation from individuals, only by means of the subjective consideration of it by our minds (53, 3); such is the solution of the problem of the universals. This teaching, which supplements the metaphysics of Heraclitus by that of Parmenides, is the very antithesis of Platonism, and we must recognize in it one of the highest achievements of the peripatetic philosophy.

Besides the common essential notes or attributes which we find repeated in all the individuals of the same species, each individual being possesses its own proper characteristics: these affect it in its essence, and give it the stamp of individuality. Between the common essence and the individualized essence there is a relation of determinable to determinant, of potency to act; thus an organic bond is seen to unite the two main theories of the peripatetic school.

The Principle of Individuation, in the things of Nature, is not the form: this of itself tends to realize the fulness of its act, and would exhaust its formative causality were it not prevented by the matter. It is the matter that individualizes the being, for it is at once quantified and limited (42); and besides, it fixes, for each being, the share of determinable reality demanded by the determinative power of the form.