That is called SIMPLE MOTION, which is excited in a body by a single cause. COMPOUND MOTION, that which is produced by two or more different causes; whether these causes are equal or unequal, conspiring differently, acting together or in succession, known or unknown. Let the motion of beings be of whatsoever nature it may, it is always the necessary consequence of their essence, or of the properties which compose them, and of those causes of which they experience the action. Each being can only move and act after a particular manner; that is to say, conformably to those laws which result from its peculiar essence, its particular combination, its individual nature: in short, from its specific energies, and those of the bodies from which it receives an impulse. It is this that constitutes the invariable laws of motion: I say _invariable_, because they can never change, without producing confusion in the essence of things. It is thus that a heavy body must necessarily fall, if it meets with no obstacle sufficient to arrest its descent; that a sensible body must naturally seek pleasure, and avoid pain; that fire must necessarily burn, and diffuse light. Each being, then, has laws of motion, that are adapted to itself, and constantly acts or moves according to these laws; at least when no superior cause interrupts its action. Thus, fire ceases to burn combustible matter, as soon as sufficient water is thrown into it, to arrest its progress. Thus, a sensible being ceases to seek pleasure, as soon as he fears that pain will be the result. The communication of motion, or the medium of action, from one body to another, also follows certain and necessary laws; one being can only communicate motion to another, by the affinity, by the resemblance, by the conformity, by the analogy, or by the point of contact, which it has with that other being. Fire can only propagate when it finds matter analogous to itself: it extinguishes when it encounters bodies which it cannot embrace; that is to say, that do not bear towards it a certain degree of relation or affinity.