Synergia — "joint working" — is mutual cooperation. For the Stoics, synergia is the teleological characterisation of human nature: human beings have been brought into being (γεγόναμεν) for one another, and they attain the fullness of their existence only in cooperation — as the organs of a single body. The concept binds individual ethics to social ethics: virtue is impossible in isolation; it is always an action within the common body of rational beings.

SVF III 340–348 (the doctrine of the social nature of the human being); Cic. De fin. III 62–66; Sen. Ep. 95.52 ("we are members of one great body"); Epict. Disc. 2.10.3–4. In Marcus: Med. 2.1; 05-16; 06-14 (cooperate with one's kind — τῷ ὁμογενεῖ συνεργεῖ); 06-42 (all are fellow-workers to one end; even the fault-finder and resister cooperate); 06-43 (the sun and stars — different, yet cooperating to one end); 07-13 (rational beings framed πρὸς μίαν συνεργίαν, like members of one body); 07-19 (bodies in the torrent — συνεργὰ τῷ ὅλῳ, cooperating with the Whole); 8.34; 09-40 (if the gods can συνεργεῖν with men at all, they can co-operate toward freedom from fear and desire); 09-42 (man is born εὐεργετικός: having done something συνεργητικόν, he already "has what is his own"); 11.8.

The famous image of 02-01: "we have been brought into being for cooperation, like feet, like hands, like eyelids, like the rows of upper and lower teeth" (see body-parts, cooperation). The opposite of synergia is counter-action (ἀντιπράσσειν): the irritation and estrangement in 02-01 are qualified precisely as counter-action — that is, as a violation of the TERMnatural order. Cf. Med. 8.34: a human being severed from the whole is like a hand or foot cut off, a dead member fallen away from nature.