Chapter XXIII.

Of Motion.

Plato and Pythagoras say that motion is a difference and alteration in matter. Aristotle, that it is the actual operation of that which may be moved. Democritus, that there is but one sort of motion, and it is that which is vibratory. Epicurus, that there are two species of motion, one perpendicular, and the other oblique. Herophilus, that one species of motion is obvious only to reason, the other to sense. Heraclitus utterly denies that there is anything of quiet or repose in nature; for that is the state of the dead; one sort of motion is eternal, which he assigns to beings eternal, the other perishable, to those things which are perishable.