I perceive two sorts of motions of bodies, acquired motion and spontaneous or voluntary motion.  In the first the cause is external to the body moved, in the second it is within.  I shall not conclude from that that the motion, say of a watch, is spontaneous, for if no external cause operated upon the spring it would run down and the watch would cease to go.  For the same reason I should not admit that the movements of fluids are spontaneous, neither should I attribute spontaneous motion to fire which causes their fluidity. [Footnote:  Chemists regard phlogiston or the element of fire as diffused, motionless, and stagnant in the compounds of which it forms part, until external forces set it free, collect it and set it in motion, and change it into fire.]

You ask me if the movements of animals are spontaneous; my answer is, “I cannot tell,” but analogy points that way.  You ask me again, how do I know that there are spontaneous movements?  I tell you, “I know it because I feel them.”  I want to move my arm and I move it without any other immediate cause of the movement but my own will.  In vain would any one try to argue me out of this feeling, it is stronger than any proofs; you might as well try to convince me that I do not exist.

If there were no spontaneity in men’s actions, nor in anything that happens on this earth, it would be all the more difficult to imagine a first cause for all motion.  For my own part, I feel myself so thoroughly convinced that the natural state of matter is a state of rest, and that it has no power of action in itself, that when I see a body in motion I at once assume that it is either a living body or that this motion has been imparted to it.  My mind declines to accept in any way the idea of inorganic matter moving of its own accord, or giving rise to any action.

Yet this visible universe consists of matter, matter diffused and dead, [Footnote:  I have tried hard to grasp the idea of a living molecule, but in vain.  The idea of matter feeling without any senses seems to me unintelligible and self-contradictory.  To accept or reject this idea one must first understand it, and I confess that so far I have not succeeded.] matter which has none of the cohesion, the organisation, the common feeling of the parts of a living body, for it is certain that we who are parts have no consciousness of the whole.  This same universe is in motion, and in its movements, ordered, uniform, and subject to fixed laws, it has none of that freedom which appears in the spontaneous movements of men and animals.  So the world is not some huge animal which moves of its own accord; its movements are therefore due to some external cause, a cause which I cannot perceive, but the inner voice makes this cause so apparent to me that I cannot watch the course of the sun without imagining a force which drives it, and when the earth revolves I think I see the hand that sets it in motion.