Simmias of Thebes (fl. 5th–4th century BC) was an ancient Greek philosopher, disciple of Socrates, and a friend of Cebes. In his Memorabilia, Xenophon includes him in the inner circle of Socrates' followers. He appears in Plato's Phaedo as a main discussion partner of Socrates alongside Cebes, as well as Crito, Phaedrus, and Epistle XIII. In addition to the references in Plato and Xenophon, Diogenes Laërtius mentions Simmias as the author of 23 brief dialogues, now lost, including On Philosophy and On Music. Simmias appears as a character in Plutarch's De Genio Socratis section of the Moralia. A pseudepigraphic letter from Xenophon to Simmias and Cebes is included in the Cynic epistles attributed to Socrates' followers. Two short works are also attributed to him in the Greek Anthology, a couplet on Sophocles and an epitaph on Plato.

Character in Plato's Phaedo

Simmias is one of Socrates' interlocutors in Plato's Phaedo. This is a philosophical dialogue by Plato, so the analogy presented in it by the character Simmias, although summarized here, need not reflect the views of the historical Simmias. Simmias' attunement analogy Socrates attacks Simmias's Analogy with four different arguments: Thus, Simmias' argument cannot be upheld.