He was the first author of a philosophical treatise in Greek prose, unless Pherecydes of Syros be an exception. (Themist. Orat. xxvi.) His work consisted, according to Diogenes, of summary statements of his opinions (πεποίηται κεφαλαιώδη τὴν ἔκθεσιν), and was accidentally found by Apollodorus. Suidas gives the titles of several treatises supposed to have been written by him ; but they are evidently either invented, or derived from a misunderstanding of the expressions of earlier writers.

The early Ionian philosophy did not advance beyond the contemplation of the sensible world. But it was not in any proper sense experimental ; nor did it retain under the successors of Thales the mathematical character which seems to have belonged to him individually, and which so remarkably distinguished the contemporary Italian or Pythagorean school. (Comp. Cousin, Hist. de la Phil. Lec. vii.) The physiology of Anaximander consisted chiefly of speculations concerning the generation of the existing universe. He first used the word ἀρχὴ to denote the origin of things, or rather the material out of which they were formed : he held that this ἀρχὴ was the infinite (τὸ ἄπειρον), everlasting, and divine (Arist. Phys. 3.4), though not attributing to it a spiritual or intelligent nature ; and that it was the substance into which all things were resolved on their dissolution. (Simplic. l.c.)

We have several more particular accounts of his opinions on this point, but they differ materially from each other.

According to some, the ἄπειρον was a single determinate substance, having a middle nature between water and air; so that Anaximander's theory would hold a middle place between those of Thales and Anaximenes, who deduced everything from the two latter elements respectively; and the three systems would exhibit a gradual progress from the contemplation of the sensible towards that of the intelligible (compare the doctrine of Anaximenes concerning air, Plut. de Pluc. Phil. 1.3), the last step of which was afterwards to be taken by Anaxagoras in the introduction of νοῦς. But this opinion cannot be distinctly traced in any author earlier than Alexander of Aphrodisias (apud Simpl. Phys. fol. 32, a.), though Aristotle seems to allude to it (de Coel. 3.5). Other accounts represent Anaximander as leaving the nature of the ἄπειρον indeterminate. (Diog. Laert. l.c. ; Simplic. Phys. fol. 6, a; Plut. Plac. Ph. 1.3.) But Aristotle in another place (Metaph. 11.2), and Theophrastus (apud Simpl. Phys. fol. 6, b, 33, a), who speaks very definitely and seems to refer to Anaximander's own words, describe him as resembling Anaxagoras in making the ἄπειρον consist of a mixture of simple unchangeable elements (the ὁμοιομερῆ of Anaxagoras). Out of this material all things were organized, not by any change in its nature, but by the concurrence of homogeneous particles already existing in it; a process which, according to Anaxagoras, was effected by the agency of intelligence (νοῦς), whilst Anaximander referred it to the conflict between heat and cold, and to the affinities of the particles. (Plut. ad Euseb. Praep. Evang. 1.8.) Thus the doctrines of both philosophers would resemble the atomic theory, and so be opposed to the opinions of Thales, Anaximenes, and Diogenes of Apollonia, who derived all substances from a single but changeable principle. And as the elemental water of Thales corresponded to the ocean, from which Homer makes all things to have sprung, so the ἄπειρον of Anaximander, including all in a confused unorganized state, would be the philosophical expression of the Chaos of Hesiod. (Ritter, art. Anaximander, in Ersch and Gruber's Encycl.)

In developing the consequences of his fundamental hypothesis, whatever that may really have been, Anaximander did not escape the extravagances into which a merely speculative system of physics is sure to fall. He held, that the earth was of a cylindrical form, suspended in the middle of the universe, and surrounded by water, air, and fire, like the coats of an onion; but that the exterior stratum of fire was broken up and collected into masses; whence the sun, moon, and stars ; which, moreover, were carried round by the three spheres in which they were respectively fixed. (Euseb. l.c. ; Plut. de Plac. 2.15, 16; Arist. de Coel. 2.13.)

According to Diogenes, he thought that the moon borrowed its light from the sun, and that the latter body consisted of pure fire and was not less than the earth; but the statements of Plutarch (de Plac. 2.20, 25) and Stobaeus (Ecl. 1.26, 27) are more worthy of credit; namely, that he made the moon 19 and the sun 28 times as large as the earth, and thought that the light of the sun issued through an orifice as large as the earth; that the moon possessed an intrinsic splendour, and that its phases were caused by a motion of rotation.

For his theory of the original production of animals, including man, in water, and their gradual progress to the condition of land animals, see Plut. de Plac. 5.19; Euseb. l.c. ; Plut. Sympos. 8.8; Orig. Phil. 100.6; and compare Diod. 1.7. He held a plurality of worlds, and of gods; but in what sense is not clear. (Cic. de Nat. Deor. 1.10 ; Plut. de Plac. 1.7.)

Anaximander flourished in the time of Polycrates of Samos, and died soon after the completion of his 64th year, in Ol. 58.2 (B. C. 547), according to Apollodorus. (apud Diog. l.c.) But since Polycrates began to reign B. C. 532, there must be some mistake in the time of Anaximander's death, unless the elder Polycrates (mentioned by Suidas, s. v. Ἴβυκος) be meant. (Clinton, Fast. Hell.) (For the ancient sources of information see Preller, Hist. Philosoph. Graeco-Romanae ex fontium locis contexta.)