Cleanthes of Assos

c. 331-232 B.C.

Greek Stoic philosopher who instigated campaigns of popular resentment against Heraklides of Pontus (c. 390-c.

310) and Aristarchus of Samos (c. 310-c. 230). Cleanthes attacked Heracleides for claiming that Earth rotated about its own axis; and he charged Aristarchus with sacrilege for proposing a heliocentric cosmology in which the world was displaced from its rightful place at the center of the universe. Cleanthes's main contribution to Stoic physics was the introduction of the concept of tonos (tension).