In these correlatives, the notion underlying the term “actual” is manifestly the notion of something completed, achieved, perfected—as compared with that of something incomplete, imperfect, determinable, which is the notion of the potential. Hence the notions of _potentia_ and _actus_ have been extended widely beyond their primary signification of power to act and the exercise of this power. Such pairs of correlatives as the determinable and the determined, the perfectible and the perfected, the undeveloped or less developed and the more developed, the generic and the specific, are all conceived under the aspect of this widest relation of the potential to the actual. And since we can distinguish successive stages in any process of development, or an order of logical sequence among the contents of our concept of any concrete reality, it follows that what will be conceived as an _actus_ in one relation will be conceived as a _potentia_ in another. Thus, the disposition of any faculty—as, for example, the scientific habit in the intellect—is an _actus_ or perfection of the faculty regarded as a _potentia_; but it is itself a _potentia_ which is actualized in the _operation_ of actually studying. This illustrates the distinction commonly drawn between an “_actus primus_” and an “_actus secundus_” in any particular order or line of reality: the _actus primus_ is that which presupposes no prior actuality in the same order; the _actus secundus_ is that which does presuppose another. The act of knowing is an _actus secundus_ which presupposes the cognitive faculty as an _actus primus_: the faculty being the _first_ or fundamental equipment of the soul in relation to knowledge. Hence the child is said to have knowledge “_in actu primo_” as having the faculty of reason; and the student to have knowledge “_in actu secundo_” as exercising this faculty. The _actus_ or perfecting principles of which we have spoken so far are all conceived as presupposing an existing subject on which they supervene. They are therefore _accidents_ as distinct from _substantial constitutive_ principles of this subject; and they are therefore called _accidental_ actualities, _actus_ “_accidentales_”. But the actual existence of a being is also conceived as the complement and correlative of its essence: as that which makes the latter actual, thus transferring it from the state of mere possibility. Hence existence also is called an _actus_ or actuality: the _actus_ “_existentialis_,” to distinguish it from the existing thing’s activities and other subsequently acquired characters. In reference to these existence is a “first actuality”—“_Esse est actus primus_”; “_Prius est esse quam agere_”: “Existence is the first actuality”; “Action presupposes existence”—while each of these in reference to existence, is a “second actuality,” an _actus secundus_.