In this paper I focus on the relationships between language, definition and being in Antisthenes. I start from Plato’s Sophist 251b–c, in which the reference to the ὀψιμαθεῖς stands out, and I conclude that it is not possible to identify these characters with Antisthenes. The conception of ὀψιμαθεῖς provides for the exclusive legitimacy of identical judgments, exploiting in an eristic sense an evident Eleatic legacy. But this position, rather than concordances, reveals serious opposition to what is surely known to us (...) of the logical and linguistic doctrines of Antisthenes. The most explicit testimony about the relationship between language and being in Antisthenes is handed down by Porphyry, who attributes to Antisthenes the equation λέγειν = λέγειν τι = λέγειν τὸ ὄν. My analysis intends to show that this equation is assumed by Antisthenes through a Socratic mediation and has a double objective: to ensure an objective reference of language, as opposed to Gorgias’ self-referential conception of language, and to nullify the antilogy, which is typical of sophistic argumentation. To it Antisthenes contrasts the ἐπίσκεψις τῶν ὀνομάτων, which is a development of Socratic ἐξετάζειν, and whose aim is to reach, at the end of the examination, no longer just a ὁμολογία, but a definition valid once and for all. Antisthenes is aware that there is a distinction between essence and quality, and with this, he moves a first important step beyond the univocal conception of being proper to Parmenides. The doxography will sum up Antisthenes’ position by describing him as the first philosopher to advance a definition of logos, i. e., definition. He bases his theory of definition on the identification of that attribute which is ἴδιον καὶ οἰκεῖον of the proposed object. Antisthenes coined the phrase τὸ τί ἦν to designate what the predicate is meant to say in a definition, i. e., the proper quality. This expression does not seem to identify the general notion of a predicate as such, but rather to indicate the determination or qualification necessary to actually define the object in question. In the final part of my article, I examine some testimonies that show how, with his theory of definition, Antisthenes is, and was considered already in antiquity, the precursor of the Stoic theory of definition, whose objective is, according to Chrysippus’ formula, to provide the explanation of what is proper (ἰδίου ἀπόδοσις). (shrink)