Page:The System of Nature.djvu/37

 s of bodies escape our senses; we know them only in the mass: we are neither acquainted with their intimate combination, nor the proportion of these combinations; from whence must necessarily result their mode of action, their impulse, or their different effects.

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.&mdash;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&mdash;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