151. Consequently, examining the question a priori, or considering the idea of body, we can find no reason for denying the possibility of its being active. It is true that the extension of bodies, inasmuch as extension, is presented to us as something without life, indifferent to all figures and to all motions, and that we do not discover in it any principle of activity;[92] but this can prove nothing, unless we suppose that the essence of bodies consists in extension, and that extension contains nothing more than is presented to our senses, that it includes nothing on which its activity can be founded. The first is an opinion, but one without any foundation; the second can never be demonstrated, because it escapes all observation, and cannot be the object of investigations a priori.
152. How can it be proved that the essence of bodies consists in extension?[93] What we may say is, that we experience it, and that all corporeal nature is presented to us under the form of extended. If we assert any thing more than this we do so without any foundation, we substitute for the reality a play of our fancy. The essence of any thing is that which constitutes it what it is, that which serves as the internal ground or root of the properties: who can say that we know this ground, this root, in corporeal objects? Our senses, it is true, perceive nothing not extended: we cannot conceive to what bodies would be reduced if deprived of extension; but from this we can only infer that extension is a form under which bodies are presented to our senses; that this form is a necessary condition of the affection of our sensibility; but not that the form is the essence of the thing, not that there is in the object nothing more intimate in which the form itself has its root.
153. If the essence of bodies consisted in extension, such as it appears to our senses, extension being equal there would be equality of essence; the essences of bodies might be measured like their dimensions; two globes of equal diameters, would be two essentially equal bodies. Experience, and even common sense are opposed to this. It may be said, that pure dimension, in so far as subject to measure, is not enough to form equality of essence; but that the equality of nature of the extension of both bodies is also requisite; but what, I ask, is the meaning of the nature of extension? If the word nature here means any thing, it must mean something distinct from extension, in so far as subject to our sensibility; in which case I infer that just as in order to diversify the essences of bodies something is imagined which is not contained in extension in so far as subject to sensible intuition, something may in the same manner be supposed which is capable of activity, and which offers to our understanding an accessory idea giving life, so to speak, to the dead matter which we find in extension, considered as the simple object of purely geometrical ideas.
154. Experience cannot demonstrate the impossibility of the activity of bodies. Absolute inactivity cannot affect us, and therefore cannot be known by experience. We can only experience action, or the exercise of activity; inaction, or the state of an absolutely inactive thing, cannot be the object of experience without a contradiction.
[CHAPTER XV.]
CONJECTURES AS TO THE EXISTENCE OF CORPOREAL ACTIVITY.
155. Experience, far from authorizing us to infer the absolute inertness of bodies, on the contrary inclines us to believe that they are endowed with activity. Although the senses do not give us intuition of any corporeal activity, they present a continuous series of changes in a fixed order in the phenomena of the corporeal world; and if the true activity of some on others can be inferred from the coincidence of their relations in space and time, from the constant succession in which we see some follow others, and the invariable experience that the existence of some suffices for the existence of others; then we must admit true activity in bodies. Whatever this argument may be worth at the tribunal of metaphysics, it has always been sufficiently powerful to convince the majority of mankind, and hence it is that the denial of the activity of bodies is contrary to common sense.
156. If we consider our relations to the corporeal world, we are equally led to believe that there is true activity in bodies. Whatever may be our ignorance of the manner in which sensations are produced within us, it is certain that we experience them in the presence of bodies which are connected with us in space and time, and in a fixed and constant order, which authorizes us to prognosticate with safety what will follow in our senses if such or such bodies are placed in relation with our organs. The idea of activity presents to us the idea of a principle of changes in other beings; bodies are continually producing real or apparent changes in us. The exercise of the sensitive faculties implies a communication with corporeal beings; in this communication the sensitive being receives from bodies a multitude of impressions causing continual changes.
157. It is said that experience shows bodies to be indifferent to rest or motion, and some works on physics at the very beginning lay it down as a thing beyond all doubt, that a body placed at rest would remain in the same state for all eternity, and if put in motion it would move for all eternity in a right line, and always with the same velocity which it at first received. I do not know how they could have learned this from experience; and I maintain that not only they could not know it, but experience seems to prove directly the contrary.