=176.= In almost every particular we find a sharp contrast between the Stoic conception of ‘body’ and the Epicurean ‘atom.’ The atom is extremely small and entirely unchangeable; ‘body’ is immensely large and in a high degree plastic. Atoms alternate with void; but ‘body’ spreads continuously throughout the entire universe; it can never be torn apart or show a gap[22]. Atoms move downwards in parallel straight lines; ‘body’ moves from the centre to the circumference, and thence returns to the centre. Two atoms can never occupy the same space; but ‘body’ everywhere moves through body, penetrating it and combining with it throughout its whole extent[23]. The atom is a convenient hypothesis within the range of modern physical and chemical science; the conception of ‘body’ gains force as we enter the region of biology. For life also is a movement which proceeds from a warm centre (and warmth is body rarefied), and extends towards a circumference which is in comparison gross and cold[24]. Going further, we find that ‘body’ and its functions are so interpreted as to provide a key to the activities of the human reason and will. [Sidenote: Tone or tension.] =177.= To the central conception of body are attached in the Stoic system various supplementary conceptions, which serve to bring into clearer view its nature and powers. Of these the most characteristic is that of ‘tone’ or ‘strain’ (τόνος, _intentio_). This term appears originally to have expressed muscular activity[25], and was next used by the Cynics to denote that active condition of the soul which is the true end of life; ‘no labour,’ said Diogenes, ‘is noble, unless its end is tone of soul[26].’ Although we cannot trace the term ‘tone’ directly to Zeno, we find that he explains sleep as a relaxation of the soul, substantially agreeing with later writers who call it a ‘relaxation of the sensory tone around the soul[27].’ With Cleanthes the word becomes fairly common, first in the ethical application, in which ‘tone’ is ‘a shock of fire, which if it be strong enough to stir the soul to fulfil its duties is called strength and force[28],’ and then in physics to explain the unceasing activity of the universe[29], personified by Hercules in Stoic allegorical theology[30]. In later writers tone becomes constantly associated with the ‘spirit’ or ‘thrill’ which explains both the unity and the movement of all things[31], so that ‘tone of spirit’ or ‘thrill-tone’ (πνευματικὸς τόνος, _intentio spiritus_) explains to us the operations of body and mind alike[32]. [Sidenote: The seed power.]