Cicero’s epistolary corpus is still partly unexplored from a philosophical angle. Modern scholars have left aside discrete and fragmentary allusions to philosophy, though the letters are a laboratory in which the origins and the development of Cicero’s thought appear more clearly than in his later works. The study of Greek words loaded with philosophical connotations, especially when these words are not translated, is particularly enlightening from this point of view. In this chapter, I successively study three different uses of philosophical Greek in Cicero’s letters: (1) Greek language betraying the influence of a philosophical model on the letters (the influence of protreptic) long before the Hortensius was written in 45 bce; (2) Greek language coming from implicit quotes, whether they serve a purely philosophical purpose or interweave philosophy and literature; (3) Greek language revealing the progressive elaboration of a philosophical work, De finibus, and its analysis of the Stoic theory of οἰκείωσις in book 3.