Though the learned men of the ancient world were almost universally agreed that the earth is a globe, they were not unanimous in the belief that it stands immovable in the center of the universe; yet the various theories which diverged from this orthodox view had no place in the development of medieval cosmology until long after our period.[39] Certain among the Pythagoreans maintained that there is a fire in the heart of the earth.[40] Plato said that the center of the earth, which stands immobile[41] in the center of the universe, is the seat, not of a fire, but of the World Soul.[42] Through its own internal movement the World Soul causes the movement of the universe as a whole. Belief in the World Soul of Plato was extraordinarily tenacious, and it emerges in the writings of more than one Neoplatonist of the Middle Ages. Aristotle, however, though he likewise held fast to the doctrine of the immobility of the earth in the center of the universe, differed both from the Pythagoreans and from Plato in refusing to believe that the center of the universe could be the seat of an incorruptible being of the same substance as the celestial bodies, be it fire or World Soul. Aristotle,[43] Pliny,[44] and Ptolemy[45] also brought forward proofs of varying validity in favor of the immobility of the earth. CIRCUMFERENCE OF THE EARTH Several figures were given by ancient authors for the circumference of the earth. Aristotle stated it to be 400,000 stades;[46] Eratosthenes determined it to be 252,000 stades according to the testimony of many writers, including Pliny,[47] Vitruvius,[48] Martianus Capella,[49] and Macrobius,[50] although Cleomedes, who gives the most circumstantial account of Eratosthenes’ measurement, had said that the latter’s figure was 250,000.[51] It is probable that Eratosthenes himself arbitrarily added 2000 stades to his result in order to obtain a figure more easily divisible.[52] Cleomedes quotes Posidonius as giving 240,000 stades,[53] and Strabo says that the latter gave 180,000 stades.[54] The last number was that adopted by Marinus of Tyre and by Ptolemy. Though we have several distinct figures cited by ancient writers, these assuredly do not indicate that as many distinct processes of measurement were carried out. The circumference given by Aristotle was a mere estimate; Eratosthenes’ result was the only one based on accurate measurements and calculations;[55] the two figures given by Posidonius may well have been derived from Eratosthenes, the larger arising from a mistaken interpretation or intentional alteration of the latter’s figure, and the smaller from the use of a longer stade.[56]