What has just been stated is, in my belief, the theory of the Platonic Timæus, signified in the words of that dialogue, and embodied in the comment of Aristotle. The commentators, subsequent to Aristotle, so far as we know them, understood the theory in a sense different from what Plato intended. I think we may see how this misconception arose. It arose from the great development and elaboration of astronomical theory during the two or three generations immediately succeeding Plato. Much was added by Eudoxus and others, in their theory of concentric spheres: more still by others of whom we read in Cicero (Academ. II. 39.) "Hicetas Syracusius, ut ait Theophrastus, coelum, solem, lunam, stellas, supera denique omnia, stare censet, neque praeter terram rem ullam in mundo moveri: quae cum circum axem se summâ celeritate convertat et torqueat, eadem effici omnia, quae si stante terrâ coelum moveretur. Atque hoc etiam Platonem in Timaeo dicere quidam arbitrantur, sed paullo obscurius." The same doctrine is said to have been held by Herakleides of Pontus, the contemporary of Aristotle, and by others along with him. (Simplikius ad Aristot. Physic. p. 64--De Coelo, p. 132--Plutarch. Plac. Phil. III. 13.) The doctrine of the rotation of the Earth here appears along with another doctrine--the immobility of the sidereal sphere and of the celestial bodies. The two are presented together, as correlative portions of one and the same astronomical theory. There are no celestial revolutions, and therefore there is no solid celestial axis. Moreover, even Aristarchus of Samos (who attained to a theory substantially the same as the Copernican, with the double movement of the Earth, rotation round its own axis, and translation round the sun as a centre) comes within less than a century after Plato's death.