Turning from the questions which have hitherto engaged our attention to natural science in the stricter sense of the term, we must first touch upon [A. The most general ideas on nature.] a few characteristic questions affecting the general conditions of all existence. In these the Stoics hold little that is of a distinctive character. The matter or substance of which all things are made is corporeal. [431] All that is corporeal is infinitely divisible, although it is never infinitely divided. [432] At the same time, all things are exposed to the action of change, since one material is constantly going over into another. [433] Herein the Stoics follow Aristotle, in contrast to the mechanical theory of nature, [434] and distinguish change in quality from mere motion in space. They enumerate several varieties of each kind. [435] Nevertheless, they look upon motion in space as the primary form of motion. [436] Under the conception of motion, they, moreover, include action and suffering. [437] The condition of all action is contact; [438] and since the motions of different objects in nature are due to various causes, and have a variety of characters, the various kinds of action must be distinguished which correspond with them. [439] In all these statements there is hardly a perceptible deviation from Aristotle. Of a more peculiar character are the views of the Stoics as to the intermingling of substances, to which reference has already been made. [440] With regard to Time and Space, they found some innovations on Aristotle’s theory to be necessary. Space (τόπος), according to their view, is the room occupied by a body, [441] the distance enclosed within the limits of a body. [442] From Space they distinguish the Empty. The Empty is not met with in the universe, but beyond the universe it extends indefinitely. [443] And hence they assert that Space is limited, like the world of matter, and that the Empty is unlimited. [444] Nay, not only Space, but Time also, is by them set down as immaterial; [445] and yet to the conception of Time a meaning as concrete as possible is given, in order that Time may have a real value. Zeno defined Time as the extension of motion; Chrysippus defines it, more definitely, as the extension of the motion of the world. [446] The Stoics affirm the infinite divisibility of Time and Space, [447] but do not appear to have instituted any deep researches into this point. [B. Elements.]