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For Condillac, algebra is the best model of knowledge. It shows how mental operations come to use signs and conversely how the use of signs develops mental operations. The “well-formed language” of algebra constitutes the central object of study in Condillac’s last work, La Langue des calculs. Left incomplete at the time of his death, La Langue des calculs seems to combine the results of the Logique with those of the Grammaire of the Cours d’études. My aim in this article is to contextualize Condillac’s definitions of algebra and analysis and to highlight that the translation procedure supposes a theory of the identity of true propositions (comparable to that of Leibniz), as well as a theory of the analogy of the different languages of mathematics.