We may look at the essence of any being from two points of view. If we consider it as it is conceived actually to exist in the being, we call it the _physical essence_. If we consider it after the manner in which it is apprehended and defined by our intellects through generic and differentiating concepts, we call it the _metaphysical essence_. Thus, the essence of man conceived by the two defining concepts, “rational animal,” is the metaphysical essence; the essence of man as known to be composed of the two really distinct substantial principles, soul and body, is the physical essence. Understood in this way both are one and the same essence considered from different points of view—as existing in the actual order, and as conceived by the mind.(87)
The physical essence of any being, understood as the constitutive principle or principles from which all properties spring, is either _simple_ or _composite_ according as it is understood to consist of one such constitutive principle, or to result from the substantial union of two constitutive principles, a material and a formal. Thus, the essence of God, the essence of a purely spiritual being, the essence of the human soul, are physically simple; the essence of man, the essences of all corporeal beings, are physically composite.
According to our mode of conceiving, defining and classifying essences by means of the abstract generic and differential grades of being which we apprehend in them, all essences, even physically simple essences, are conceived as logically and metaphysically composite. Moreover we speak and think of their generic and differential factors as “material” and “formal” respectively, after the analogy of the composition of corporeal or physically composite essences from the union of two really distinct principles, matter and form; the analogy consisting in this, that as matter is the indeterminate principle which is determined and actuated by form, so the _generic_ concept is the indeterminate concept which is made definite and specific by that of the _differentia_.(88) But when we think of the _genus_ of any corporeal essence as “material,” and the _differentia_ as “formal,” we must not consider these “metaphysical parts” as really distinct; whereas the “physical parts” of a corporeal substance (such as man) are really distinct. The genus (animal), although a metaphysical part, expresses the _whole essence_ (man) in an indeterminate way; whereas the “matter” which is a physical part, does not express the whole essence of man, nor does the soul which is also a physical part, but only both together. Not a little error has resulted from the confusion of thought whereby _genus_ and _differentia_ have been regarded as material and formal constitutives in the literal sense of those expressions.