ESSENCE. - I. It indicates the nature of a thing as such and thus what is signified in its definition. If the existent (v.) is the concrete subsisting thing that exists or has the act of being, the essence is the specific nature (οὐσία) that this act causes to exist. The essence expresses in its content a “participation” in the infinite perfection of the divine nature; it determines for each being its own place in the hierarchy of beings; it founds, directs, and actualizes their respective possibilities of development. As an expression of the fundamental heterogeneity of the real, essences are fixed and immutable both in time and in space: they succeed one another in a discontinuous manner, through leaps that are distinct degrees of perfection within the realm of being: as lead, oak, horse, man... Aristotle accepted, with appropriate refinements, the tradition of Platonic-Pythagorean intellectualism “that essences are.” In a certain way, as number (Met., IX, 3, 1043 b 32 sqq.). Thus essences at once found the stability of being and the objectivity of knowing: hence the Neo-Latin term “quiddity” (quidditas = “quid sit aliquid”). Essence is said both of substance and of accident, but primarily of substance.
In Christian thought, which does not accept creation “ab aeterno,” such perpetuity has purely ideal significance because in fact the world has had a beginning, and within it every creature—and thus every essence—under the aspect of existential duration has a value corresponding to its own nature: of finite duration if the nature is contingent (e.g., material and corruptible), of infinite duration (and essences) a posteriori if the nature is spiritual (angels and human souls). V. ANIMA; FORMA.