It may be said, that in solid bodies, which follow the laws of mechanism, the re-action is always equal to the action; but that in the animal body it appears that the re-action is greater than the action, and that the other exterior movements ought not to be considered as simple effects of the impression of objects upon the senses. To this objection I reply, that though in certain cases effects appear proportioned to their causes, there is in Nature an infinite number of cases where the effects bear no kind of proportion to their apparent causes. By a single spark of fire a magazine of powder may be set in flame, and a citadel be blown up. By electricity a slight friction produces a violent shock, which is communicated to great distances, and if a thousand persons touch each other, they would all be almost as much affected by it as if the shock had been confined to each of them individually. It is not, then, extraordinary that a slight impression on the senses should produce in the animal body a violent re-action, and should manifest itself by exterior movements. The causes we are qualified to ascertain, and the quantity of whose effects we can precisely estimate, are less numerous than those whose mode of action is unknown, and of whose proportional relation with their effects, we are entirely ignorant. Now most effects in Nature depend on a number of causes differently combined, whose actions vary, and seem to be determined by no established law, consequently we can only form a conjectural estimate by endeavouring to approximate the truth by the means of probabilities. I pretend not, then, to assert as a demonstrative fact, that progressive and other exterior movements of animals, are caused solely by the impression of objects upon the senses. I mention it merely as likely, and founded on principles of analogy, since all organized beings, which are destitute of sense, are likewise destitute of progressive motion, and that all those which possess the one have also the other.