Aquinas accepts the general principles of the metaphysics of Aristotle, for whom there are two principles of being, potency and act.
Act signifies being, reality, perfection; potency is non-being, non-reality, imperfection.
Potency does not, however, mean absolute non-being, but rather the capacity to receive some perfection, the capacity to exist, as Aristotle taught.
The transition from potential to actual existence is becoming, that is, the passage from potency to act. Outside of becoming there exists Pure Act, the absolute reality and perfection upon which all becoming depends.
The general principle of metaphysics, potency and act, applied to that part of becoming in which matter is already existent, is specified in a second principle, the principle of matter and form.
Matter which in potency is not be understood as pure nothingness, but is as a being having in itself no determination. Thus matter is to be conceived of as the substratum of form. The form which is in act gives to the matter specific determination, reality, perfection — that which we mean when we ask what is such and such a thing.
The union of matter and form constitutes or gives place to the substance, to the “totum,” the individual. Relative to the question of the principle of individuation, or the question of how it happens that a determined specific form can give place to a multiplicity of individuals of the same species, Aquinas affirms that the principle of individuation is matter — not matter considered abstractly, pure matter, but matter signed by quantity, or that concrete matter in which the new form is produced.
If prime matter and substantial form are sufficient to constitute the “totum” (the substance), then this latter, to be perfect, can and must receive other or secondary forms, i.e., accidental forms which give new determination to the substance (quantity, quality, etc.). The accidents, since they are determinations of the substance, are ordained to the substance and depend on it.
The concept of matter and form gives us an explanation of how a thing becomes, but does not tell why it becomes. To present us with the why of becoming, it is necessary to have recourse to a third concept — that of efficient cause — which produces such a determination of form in matter and is the reason why this particular form arises in the matter.
Finally, to give us the reason why the efficient or acting cause or agent is made to bring about the union of this form in this matter, we need a fourth element, the concept of end. End (finis) indicates the purpose the agent has in mind when he acts, or gives actuation to this form in this matter.
Final cause hence indicates the end, and also the order according to which the agent is determined to act: First in intention, the purpose or end is last in execution — the purpose of the agent is achieved only when the entity is completed in its material element and its substantial and accidental forms.
Thus for Aquinas, as for Aristotle, the concepts explaining reality are reduced to the concepts of the four causes — material, formal, efficient, and final.