_Act_, in the scholastic language, is that which gives existence by formal actuation. _Potency_ is that which, by formal actuation, receives existence. _Actuality_ is the result of the actuation--that is, the very existence of the act in its potency. Actuality, as we have already remarked, was also called _actus existentiæ_; hence, existence itself was considered as an act received in the essence, and causing it to be. But this view is now generally abandoned, because it has been shown that it is not the existence that entails the reality of the act and the potency, but the real position of the act in its potency that entails the existence of the being. Accordingly, existence is not an act received in the essence, but the result of the position of the essence; and cannot be called _an act_, except in a logical sense, inasmuch as it gives to the being _denominationem existentis_. An act is called _essential_ when it gives the first existence to any essence, be it simple or compound; _substantial_, when it gives the first existence to a pure potency; _accidental_, when it gives a mode of being. The distinction between essential and substantial acts will be explained here below, where we examine the different kinds of forms. Every being acts inasmuch as it is in act, and is acted on inasmuch as it is in potency. Hence, the substantial act is a principle of activity, and the potency a principle of passivity. The active power of any being, if taken in the concrete, is nothing but its substantial act _as ready for exertion_, and is called active power, because its exertion is the position or the production of an act. The active power thus considered is, therefore, in reality one of the constituent principles of natural beings; whilst the abstract term _activity_ does not stand for a principle, but for an attribute of the being--that is, for its _readiness to act_. The passive potency of any being, if taken in the concrete, is nothing but the term of the substantial act _as liable to be acted on_, and is called passive or receptive, because it is actuated by the reception of an act. The passive potency, thus considered, is therefore in reality one of the constituent principles of natural beings, whilst the abstract term _passivity_ does not stand for a principle, but for an attribute of the being--that is, for its _liability to be acted on_. Every one who is acquainted with metaphysical matters will acknowledge that it is of extreme importance that these terms and others of a like nature, which are continually employed in metaphysical analysis, be clearly understood by all students of philosophy. So long as our language has no definite words by which to designate the essential constituents of things, no hope can be entertained of advancing the interests of metaphysics by means of vernacular books.