ESSENCE. - I. It indicates the nature of a thing as such and therefore of what is signified in its definition. If the "being" (v.) is the concrete subsistent that exists or that has the act of being, the e. is the special "nature" (ὁσία) that this act causes to exist. The e. 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, e. are fixed and immutable both in time and in space: they therefore succeed one another in a discontinuous manner, by leaps that are distinct degrees of perfection within the ambit of being: such as lead, oak, horse, man... Aristotle accepted, with appropriate rectifications, the tradition of Platonic-Pythagorean intellectualism "that the e. are in some way like numbers" (Met., IX, 3, 1043 b 32 ff.). Thus the e. found at one and the same time the stability of being and the objectivity of knowing: hence the Neo-Latin term "quiddity" (quidditas = "quid sit aliquid"). E. is said both of substance and of accident, but of substance in a principal sense and of accident in a secondary sense and according to analogy, since just as the accident, which is not so much "ens" as "ens entis", participates from substance the reason of being, so likewise its e. depends on that of its own substance, so that in its definition the accident includes substance as subject and foundation as well as radical active principle (St. Thomas, In VII Met., lect. 4, n. 1334). God properly does not have an e. but is Pure Act of being beyond all definition.
II. In its double aspect, ontological and logical, the e. admits various degrees of consideration. There is first of all the singular and individual e., the humanity of Peter and of Paul (the 268e ff.). In Aristotelian realism it is the only one that truly exists in reality, and it is therefore on it that the further determinations of scientific consideration must be founded. Since in singular substances the e. is found multiplied and individualized, Aristotle could not stop at "form" alone as Plato did, but had to introduce matter as a co-principle of the e.
Therefore sensible things have an e. composed of matter and form, the union of which gives the complete species: man, as such, is not soul only, but the synthesis of soul and body (Met., VI, 1, 1025 b 31 ff., and VIII, 4-6, 1045 b 3 ff.). Singularity, however (the "humanity" as it is found in Peter, realized with "this" soul as the act of "this" body), cannot be an attribute of the e. as such but must be referred to extra-essential principles (v. INDIVIDUATION, PRINCIPLE of).
On the rigorously metaphysical plane, the e. is properly defined as "form" (εἶδος) of being, and as such does not require the presence of matter: indeed at the summit of the scale of beings stand the wholly immaterial "forms" (pure spirits). And in the sensible world itself the e. is determined by the form; man is this e. and not another, by virtue of the spiritual soul that is his form; that the body too has a special nature is in turn a requirement and an effect of the same soul to which it is destined, since what determines is the act and not the potency. So that for Aristotle the e., in its metaphysical absoluteness, is to be called "without matter" (Met., VIII, 7, 1032 b 14). The soul (forma partis) is the act of a composite e., act of matter: "humanitas" instead expresses the reality of the whole (forma totius) which embraces both matter and form in a synthesis that is grade, mode and form of being among the other forms expressed by the other e. (St. Thomas, In VI Met., lect. 9, n. 1460).
In the logical order the e. is given by the definition (v.). As the e. has real parts, matter and form, so the definition has logical parts which are the genus and the difference, from which results the species that is the complete e. (Met., VIII, 9, 1034 b 20 ff.). Note, however, that genus and difference are "formal parts" of the being, of which the one expresses the determinable element, the other the determining element, and under this aspect only, that is indirectly, do they correspond to the composition of matter and form. Among the Scholastics, those who with Avicebron (v.) hold a direct correspondence between the logical order and the ontological order admit the composition of matter and form even in pure spirits. According to St. Thomas, on the other hand, intellectual substances are absolutely simple in the order of the e., but not in the order of being, in which they are really composed of e. and of the act of being and therefore of substance and of faculties, unlike God who is simple under every aspect (Sum. Theol., 1a, q. 54, a. 1-3).
In the delicate Aristotelian terminology, τὸ τί ἦν εἶναι is to be distinguished from τὸ ἦν εἶναι or rather τὸ ἀνθρώπῳ εἶναι: the first stands for the generic content of the e., the second indicates the e. in its full and definitive determination. The dative has a possessive meaning; the imperfect (ἦν) is brachylogical: it indicates both the precedence of the concept with respect to the concrete that is modeled on it, and the perpetuity that belongs to essential being (Trendelenburg, Schwegler, Bonitz). In the Aristotelian philosophy, which does not accept creation "ab aeterno", such perpetuity has a purely ideal meaning because in fact the world has begun and in it every creature: and therefore every e., under the aspect of existential duration, has the value corresponding to its own nature: of finite duration under every aspect, if the nature is contingent (corruptible material e.), of infinite duration (immortal e.) a parte post, if the nature is spiritual (angels and human souls). V. SOUL; FORM.