Our senses bring us generally acquainted with two sorts of motion in the beings that surround us: the one is the motion of the mass, by which an entire body is transferred from one place to another. Of the motion of this genus we are perfectly sensible.--Thus, we see a stone fall, a ball roll, an arm move, or change its position. The other is an internal or concealed motion, which always depends on the peculiar energies of a body: that is to say, on its _essence_, or the combination, the action, and re-action of the minute--of the insensible particles of matter, of which that body is composed. This motion we do not see; we know it only by the alteration or change, which after some time we discover in these bodies or mixtures. Of this genus is that concealed motion which fermentation produces in the particles that compose flour, which, however scattered, however separated, unite, and form that mass which we call BREAD. Such also is the imperceptible motion by which we see a plant or animal enlarge, strengthen, undergo changes, and acquire new qualities, without our eyes being competent to follow its progression, or to perceive the causes which have produced these effects. Such also is the internal motion that takes place in man, which is called his INTELLECTUAL FACULTIES, his THOUGHTS, his PASSIONS, his will. Of these we have no other mode of judging, than by their action; that is, by those sensible effects which either accompany or follow them. Thus, when we see a man run away, we judge him to be interiorly actuated by the passion of fear. Motion, whether visible or concealed, is styled ACQUIRED, when it is impressed on one body by another; either by a cause to which we are a stranger, or by an exterior agent which our senses enable us to discover. Thus we call that _acquired motion_, which the wind gives to the sails of a ship. That motion which is excited in a body, that contains within itself the causes of those changes we see it undergo, is called SPONTANEOUS. Then it is said, this body acts or moves by its own peculiar energies. Of this kind is the motion of the man who walks, who talks, who thinks. Nevertheless, if we examine the matter a little closer, we shall be convinced, that, strictly speaking, there is no such thing as spontaneous motion in any of the various bodies of Nature; seeing they are perpetually acting one upon the other; that all their changes are to be attributed to the causes, either visible or concealed, by which they are moved. The will of man is secretly moved or determined by some exterior cause that produces a change in him: we believe he moves of himself, because we neither see the cause that determined him, the mode in which it acted, nor the organ that it put in motion.