something capable of being actually so and so before the change and actually otherwise after the change. For real change, therefore, we require (1) two positive and really different states of the same being, a “_terminus a quo_” and a “_terminus ad quem_”; and (2) a real process of transition whereby something potential becomes actual. In creation there is no real and positive _terminus a quo_; in annihilation there is no real and positive _terminus ad quem_; these therefore are not changes in the proper sense of the term. Sometimes, too, change is affirmed, by purely extrinsic denomination, of a thing in which there is no real change, but only a relation to some other really changing thing. In this sense when an object unknown or unthought of becomes the actual object of somebody’s thought or cognition, it is said to “change,” though the transition from “unknown” to “known” involves no real change of state in the object, but only in the knowing subject. If thought were in any true sense “constitutive” of reality, as many modern philosophers contend, the change in the object would of course be real. Since, therefore, change consists in this, that a thing which is actually in a given state ceases to be actually such and begins to be actually in another state, it is obvious that there persists throughout the process some reality which is in itself potential and indifferent to either actual state; and that, moreover, something which was actual disappears, while some new actuality appears, in this persisting potentiality. The abiding potential principle is called the _matter_ or _subject_ of the change; the transient actualizing principles are called _forms_. Not all these “forms” which precede or result from change are necessarily positive entities in themselves: they may be mere _privations_ of other forms (“_privatio_,” στέρησις): not all changes result in the acquisition of a new degree of positive actual being; some result in loss of perfection or actuality. Still, even in these cases, the state characterized by the less perfect degree of actuality has a determinate actual grade of being which is proper to itself, and which, as such, is not found actually, but only potentially, in the state characterized by the more perfect degree of actuality. When, then, a being changes from a more perfect to a less perfect state, the actuality of this less perfect state cannot be adequately accounted for by seeking it in the antecedent and more perfect state: it is not in this latter state _actually_, but only _potentially_; nor do we account for it by saying that it is “equivalently” in the greater actuality of the latter state: the two actualizing principles are really distinct, and neither is wholly or even partially the other. The significance of this consideration will appear presently in connection with the scholastic axiom: _Quidquid movetur ab alio movetur_.