As we are going to treat of essence or essences, we must before
everything else clear up the significance of essence, which we are
now considering, and distinguish it from what other people mean by
that word, which we would more likely call that which becomes, what
is never genuine essence. And besides, it must be clearly understood
that in making this distinction, we do not intend to divide a genus
in species of the same nature; as Plato tried to do.[299] For it
would be ridiculous to subsume under the same genus both essence and
non-essence, or Socrates, and the image of Socrates. The kind of
divisions here attempted will therefore only consist in separating
things essentially different, as, for instance, explaining that
apparent essence is not the same as the veritable Essence, by
demonstrating that the latter's nature is entirely different. To
clarify this its nature, it will be necessary to add to the idea of
essence that of eternity, and thus to demonstrate that the nature of
being could never be deceptive. It is of this kind of essence (that is,
of the intelligible Essence), that we are going to treat, admitting
that it is not single. Later[300] we shall speak of generation, of what
becomes, and of the sense-world.