The word "act" with us primarily signifies that which is produced by action; for all action is the production, or the position, or the making of an act. But all action implies an agent--that is, a being which is already "in act," with its actual power prepared for action. On the other hand, nothing is formally "in act," but through an intrinsic "act," which is the formal principle of its actuality. Accordingly, the word "act," though primarily known to us as expressing the product of action, must, by metaphysical necessity, be applied also to that from which every agent and every being has its actuality. Hence, philosophers found it necessary to admit two kinds of "acts"--the _essential_ and the _accidental_. The essential is that which gives the first actuality, or existence, to a being--_dat esse simpliciter_. The accidental is that which is received in a subject already existing, and which only gives it an accidental actuality or a mode of being--_dat esse secundum quid_. But the essential act (which is also called _substantial_, though it has a more extensive meaning, as we shall see hereafter) is, moreover, to be distinguished from actual existence. Metaphysicians, indeed, very often speak of existence as an act; and hence, to avoid confusion and equivocation, they are obliged to distinguish the _actus essentiæ_ from the _actus existentiæ_. Yet, to speak properly, existence is not simply an act; it is the actuality of the being;[68] and, consequently, the distinction which must be admitted between the essential act and the existence of a being is not strictly a distinction between two acts, but between the _act_ which actuates the essential term of the being, and the _actual state_ which results from such an actuation. I will say more on this point when I have explained the use of the word "potency." The English word "potency" is the equivalent of the Latin _potentia_. This Latin word, although used most frequently in the sense of "passive principle," is not, however, necessarily connected with passivity more than with activity; and accordingly it has been used as well to designate "active power." Hence, it is obvious that this term, _potentia_, when employed absolutely without the epithet _activa_ or _passiva_, is liable to two interpretations, and becomes a source of mischievous equivocations. I do not see what prevented our old Latin philosophers from designating the two kinds of _potentia_ by two different words. Had they constantly used _virtus_ or _vis_ for the _potentia activa_, and reserved _potentia_ exclusively for the _potentia passiva_, they would not have mistaken the one for the other, as they sometimes did. Let me quote a few examples of this for our common instruction.