Hylemorphism is the theory according to which the corporeal substance, essentially (per se) one, is composed of two metaphysical and substantial principles, prime matter and substantial form.³⁴ This theory is the cornerstone of the philosophy of nature. Hylemorphism is the ultimate explanation of corporeal being as such.³⁵

Life is the totality of biological phenomenon in the strict sense, so that nutrition is one essential exhibited by certain bodies showing life.³⁶ Here we do not consider spiritual life, moral life, social life, or religious life.³⁷ Life, defined more metaphysically, is that something acts immanently or moves itself. Donat gives this definition and proof: immanent action is action which proceeds from an agent and terminates in an agent while remaining in that same agent, or consists in moving one’s self; but life is such an action; therefore life is immanent action.³⁸ Life is analogous concept, applicable to plants, animals, man and God.³⁹

Cause is all that in some way contributes to the production of something.⁴⁰ Aristotle divides causes into four species: material, formal, efficient, and final. Aristotle’s attention was above all concentrated on causality since the study of the causes is the principle objective of science (Aristotle Metaphysics 1. 2. 983. 20-30).

Prime Matter is an incomplete substance, which as the determinable part, constitutes a substantial corporeal composite.⁴¹ Aristotle defines prime matter as "the first subject of any thing, the subject by which (ens quo) something becomes (substance) when it (the prime matter) is in it (the substance) but not as an accident"; thus it is a incomplete "subject"; and "first" to distinguish it from second matter which has accidents; and "when it is in" to indicate not a privation, but an intrinsic principle of the composit of the thing; and "by which...something becomes" to indicate that it is the substance and not an accident.⁴² The Scholastics have a Latin saying to remind themselves about prime matter: "Materia prima secundum se non est quid (not a substance), non est quale (not quality), non est quantum (not quantity), nec aliquid eorum quibus ens determinatur (not other accidents)"⁴³ Prime matter is important for evolution because it is the subject of substantial mutation, and the origin of a species is substantial mutation.

Second Matter is the composit of matter and form which is the subject of accidental mutation.⁴⁴

Substantial Form is an incomplete substance which as the determining part, constitutes the substantial corporeal composit.⁴⁵ Substantial form is important for evolution because it constitutes the species in a determinate manner of being.⁴⁶

Transcendental relation is the way in which matter and form are related, that is, by their very entity they tend toward each other, they need each other. No bond is required in a union like this. Matter and form are united immediately and exist by a single "to be." This is the reason the substance is called perfectly one (unum per se).⁴⁷

The Composite from each reality does not merely consist in a juxtaposition or compenetration, although this may be necessary, but in mutual communication, through which the potency of the material is realized and actuated through the form, and the form actuates the potency of the material.⁴⁸ From the union of matter and form results the substantial composit, which is the concrete being we observe in all of nature.⁴⁹

Intrinsic Change happens when the same real and physical thing intrinsically differs from what is formerly was, e.g., when cold water becomes hot.⁵⁰ For true intrinsic change requires, first, some act or perfection which is acquired or lost by the change, and second, some real subject which is able to have or lose an act; in other words, the subject of itself is indifferent to both act and its privation.⁵¹ In the example of the water just given, water is the real subject, which is passive potency to heat or lack of heat; the heat itself is the act or perfection, which is acquired or lost by change. When true mutation happens, as the subject moves from one act to another act, in order that it has a place in the conversion, the subject of the mutation must be in potency to both acts. So when someone changes from error to a knowledge of truth, there is a subject, namely the intellect, which is capable of having both acts, of error or of truth.

Substantial Change is a change that reaches even to the substantial being (esse) of the body.⁵² Such a substantial change in material bodies needs two elements, namely some true physical reality which is indeterminate in the genus of substance (called prime matter), for otherwise there would be change without a subject. The act corresponding and determining prime matter to this or that species of corporeal substance, is called substantial form. Therefore, if there is substantial change in bodies, it is necessary to conclude that corporeal bodies consist of two elements, both in the genus of substance, with one as potency and the other as act.⁵³