Even in the sixth century B.C., when the habit of composing in prose was first introduced, Pherekydes and Akusilaus still continued in their prose the theogony, or the mythical cosmogony, of Hesiod and the other old Poets: while Epimenides and the Orphic poets put forth different theogonies, blended with mystical dogmas. It was, however, in the same century, and in the first half of it, that Thales of Miletus (620-560 B.C.), set the example of a new vein of thought. Instead of the Homeric Okeanus, father of all things, Thales assumed the material substance, Water, as the primordial matter and the universal substratum of everything in nature. By various transmutations, all other substances were generated from water; all of them, when destroyed, returned into water. Like the old poets, Thales conceived the surface of the earth to be flat and round; but he did not, like them, regard it as stretching down to the depths of Tartarus: he supposed it to be flat and shallow, floating on the immensity of the watery expanse or Ocean.[6] This is the main feature of the Thaletian hypothesis, about which, however, its author seems to have left no writing. Aristotle says little about Thales, and that little in a tone of so much doubt,[7] that we can hardly confide in the opinions and discoveries ascribed to him by others.[8] [Footnote 6: Aristotel. Metaphys. A. 3, p. 983, b. 21. De Coelo, ii. 13, p. 294, a. 29. [Greek: Thalê=s, o( tê=s toiau/tês a)rchêgo\s philosophi/as], &c. Seneca, Natural. Quæst. vi. 6. Pherekydes, Epimenides, &c., were contemporary with the earliest Ionic philosophers (Brandis, Handbuch der Gesch. der Gr.-Röm. Phil., s. 23). According to Plutarch (Aquæ et Ignis Comparatio, p. 955, init.), most persons believed that Hesiod, by the word Chaos, meant Water. Zeno the Stoic adopted this interpretation (Schol. Apollon. Rhod. i. 498). On the other hand, Bacchylides the poet, and after him Zenodotus, called Air by the name Chaos (Schol. Hesiod. Theogon. p. 392, Gaisf.). Hermann considers that the Hesiodic Chaos means empty space (see note, Brandis, Handb. d. Gesch. d. Gr.-Röm. Phil., vol. i., p. 71).] [Footnote 7: See two passages in Aristotle De Animâ, i. 2, and i. 5.] [Footnote 8: Cicero says (De Naturâ Deorum, i. 10), "Thales--aquam dixit esse initium rerum, Deum autem eam mentem, quæ ex aquâ cuncta fingeret." That the latter half of this Ciceronian statement, respecting the doctrines of Thales, is at least unfounded, and probably erroneous, is recognised by Preller, Brandis, and Zeller. Preller, Histor. Philos. Græc. ex Fontium Locis Contexta, sect. 15; Brandis, Handbuch der Gr.-R. Philos. sect. 31, p. 118; Zeller, Die Philos. der Griechen, vol. i., p. 151, ed. 2.