[B. Dynamical theory of nature.] [(1) Matter and force.] Although the stamp of materialism was sharply cut, and its application fearlessly made by the Stoics, they were yet far from holding the mechanical theory of nature, which appears to us to be a necessary consequence of strict materialism. The universe was explained on a dynamical theory; the notion of force was placed above the notion of matter. To matter, they held, alone belongs real existence; but the characteristic of real existence they sought in causation, in the capacity to act and to be acted upon. [279] This capacity belongs to matter only by virtue of certain inherent forces, which impart to it definite attributes. Let pure matter devoid of every attribute be considered, the matter which underlies all definite materials, and out of which all things are made; [280] it will be found to be purely passive, a something subject to any change, able to assume any shape and quality, but taken by itself devoid of quality and unable to produce any change whatsoever. [281] This inert and powerless matter is first reduced into shape by means of attributes, [282] all of which suppose tension in the air-currents which produce them, and consequently suppose a force producing tension. [283] Even the shape of bodies, and the place they occupy in space, is, according to the Stoics, something derivative, the consequence of tension; tension keeping the different particles apart in one or the other particular way. [284] Just as some modern physiologists construct nature by putting together a sum of forces of attraction and repulsion, so the Stoics refer nature to two forces, or, speaking more accurately, to a double kind of motion—expansion and condensation. Expansion works outwardly, condensation inwardly; condensation produces being, or what is synonymous with it, matter; expansion gives rise to the attributes of things. [285] Whilst, therefore, they assert that everything really existing must be material, they still distinguish in what is material two component parts—the part which is acted upon, and the part which acts, or in other words matter and force. [286] [(2) The nature of force.] [(a) Force limited to the notion of efficient cause.]