Sphaerus

Sphaerus of Borysthenes, also known as the Bosporan, was a Stoic philosopher who lived roughly between the late fourth and the early third century BC (the sources do not agree on his exact dates). He belonged to the first generation of the school founded by Zeno of Citium, whom he followed before studying under Cleanthes. His significance lies mainly in the reputation he enjoyed among the Stoics for the precision of his definitions.

He taught in Sparta, where he advised Cleomenes III, and later moved to Alexandria during the reign of the first Ptolemies. Diogenes Laertius credits him with a long list of treatises, among them On the Universe in two books, along with works on fortune, the passions and dialectic. Very little of them survives, so his thought must be reconstructed largely from indirect references. The titles show an interest that spans physics, ethics, theory of knowledge and logic, together with a debate with the atomist theories.

His influence was that of a teacher valued for the rigour of his method rather than that of a builder of new systems. He is remembered as a respected figure within the Stoic school, although it remains difficult to reconstruct his positions in detail.