produced by parallel agents, comes to consider in one thing the possibility that one of its simple ideas may be changed, and in another the possibility of producing that change, and in that way the mind forms the idea of power.] Th is power corresponds to the Latin potentia, it is opposed to act, and the passage from power to act is change. This is what Aristotle understands by the word motion, when he says * that it is the act or perhaps the actuation of that which is in power. It may be said then that power in general is the possibility of change. Now change or the act of this possibility, being action in one subject and passion in another, there will be two powers, one passive, the other active. The active may be called faculty, and perhaps the passive might be called capacity or receptivity. It is true the active power is sometimes taken in a more complete sense, when besides the simple faculty there is a tendency; and it is thus that I take it in my dynamical considerations. The word force might be appropriated to it in particular; and force would be either entelecliy or effort; for entelecliy (although Aristotle takes it so generally that it comprises also all action and all effort) appears to me more appropriate to primitive acting forces, and that of effort to the derivative. There is even also a kind of passive power more particular and more endowed with reality ; namely, that which is in matter in which there is not only mobility, which is the capacity or receptivity for motion, but also resistance, which includes impenetrability and inertia. Entelechies, i.e. primitive or substantial tendencies, when accompanied by perception, are souls.] . Ph. The idea of power expresses some kind of relation. But what one of our ideas of whatever kind does not include some relation? Our ideas of extension, of duration, of number, do they not all contain in themselves a secret relation of parts? The same thing is noticed in a still more visible manner in figure and motion. Sensible qualities, what are they but the powers of different bodies in relation to our perception, and do they not depend in themselves upon bulk, figure, the contexture and motion of the parts? which puts a kind of relation between them. Thus our idea of power may very well be placed in my opinion among the other simple ideas. Th. [At bottom the ideas which we have just enumerated are composite; those of sensible qualities hold their place among the simple ideas only because of our ignorance, and the others