Alcmaeon of Croton was by some late writers called a Pythagorean. Aristotle, however, who thought him of sufficient importance to compose a work in refutation of him (TTposTcc ‘AAkijoucovos, D.L. v, 25), expressly distinguishes him from the Pythagoreans, and is borne out in this by some of the doctrines attributed to him. His citizenship of Croton, together with his belief in the immortality of the soul and its kinship with the divine, and in the divinity of the stars, and a general emphasis on the role of opposites in nature, would be sufficient, in the opinion of uncritical Neopythagorean and Neoplatonic writers, to warrant the Pythagorean label. In fact, however, he seems to have displayed considerable originality of thought, ‘without’, as Heidel put it, ‘recognizable affiliation with any special group of natural philosophers’.

This comparative independence makes it difficult to determine his date by assigning him a probable place, on internal testimony, in the succession of Presocratic philosophers. Of positive evidence we have only one sentence of Aristotle's Metaphysics, which is, however, missing from the Laurentian MS. Ab. In ch. 5 of the first book, Aristotle lists the ten Pythagorean pairs of opposites (p. 245, above), and continues (986327): ‘Alcmaeon of Croton appears to have spoken in the same way, and either he took this doctrine from them or they from him; for as to his period, he lived in the old age of Pythagoras.’ Admittedly the Greek of the last clause is vague (cf. Wachtler, op. cit. n. 1 on p. 342), but at least it means that the lifetimes of Pythagoras and Alcmaeon overlapped. On the other hand the words long ago came under suspicion of being a later interpolation, though the editors of Aristotle admitted them until Ross, who writes (note ad loc.) that they ‘are omitted by Ab, and there is no trace of them in Alexander; they are probably a later addition, though the statement is likely enough to be true ’.