Sphaerus (gr. Sphairos), called, apparently from the country of his birth, Bosporianos by Diogenes Laertius (vii. 177), and Borusthenites by Plutarch (Cleom. c. 2), was a philosopher of the Stoic school.

He studied first under Zeno of Citium. and afterwards under Cleanthes. He lived at Alexandria during the reigns of the first two Ptolemies, having gone there apparently at the invitation of Ptolemy Philadelphus. He also taught at Lacedaemon, and was believed to have had considerable influence in moulding the character of Cleomenes. (Plut. l. c.) He was in considerable repute among the Stoics for the accuracy of his definitions (Cic. Tusc. iv. 24. § 53). Diogenes Laertius (l. c.) and Athenaeus (viii. p. 334. e.) tell a story of the dexterous manner in which, on one occasion, by the help of his subtle distinctions, he saved himself from the necessity of admitting that he had been deceived by a trick played upon him by king Ptolemy. He was, according to Diogenes Laertius, the author of the following works and treatises :

• 1. Peri kosmou.
• 2. Peri stoicheion spermatos.
• 3. Peri tuches.
• 4. Peri elachiston (on the atomic theory).
• 5. Pros tas atomous kai ta edola.
• 6. Peri aistheterion.
• 7. Peri Herakleitou e diatribon.
• 8. Peri tês êthikês diataxeôs.
• 9. Peri kathekontos.
• 10. Peri hormes.
• 11. Peri pathon, in two books.
• 12. Diatribai.
• 13. Peri basileias.
• 14. Peri Lakonikes politeias. Athenaens (iv. p. 141 b.) quotes from the third book of this work.
• 15. Peri Lukourgou kai Sokratous, in three books. It does not appear whether it is this work or the preceding which is quoted by Plutarch (Lyc. 5.)
• 16. Peri nomou.
• 17. Peri mantikes.
• 18. Dialogoi erotikoi.
• 19. Peri ton Eretriakon philosophon.
• 20. Peri homoion.
• 21. Peri horon.
• 22. Peri hexeos.
• 23. Peri ton antilegomenon.
• 24. Peri logou.
• 25. Peri ploutou.
• 26. Peri doxes.
• 27. Peri thanatou.
• 28. Techne dialektike, in two books.
• 29. Peri kategorematon.
• 30. Peri amphibolion.
• 31. Epistles.

None of these are now extant. Diogenes Laërtius (vii. 178), mentions a treatise by Chrysippus against some of the views entertained by Sphaerus. (Fabric. Bibl. Gr. vol. iii. p. 576; Vossius, de Hist. Graec. p. 140; Schöll Gesch. der Griech. Litt. vol. ii. p. 216.)