According to Irving Lavin, the renowned scholar of Bernini, the artist “was not a theoretician”. Still recently the sculptor has been confronted with such artists of clearly articulated intellectual ambitions as Domenichino, Pietro Testa and Nicolas Poussin. Even though this statement in principle is correct, it is also true that the rigours and significance of the theoretical foundations of the artistic practice of Domenichino and Poussin, just like those of Bernini, can be inferred, above all, from their œuvre. What can be verified is whether, and how clearly and coherently those theoretical foundations became verbally formalized by the artists themselves. The paramount importance of such principles of Bernini’s art as the “speaking likeness”, and above all, the bel composto, which had many precedents in the history of art but have never been dealt with in Renaissance art theory, has not been sufficiently emphasized from the viewpoint of theoretical originality which can be inferred from the artist’s biography and Chantelou’s diary. Even more remarkable are Bernini’s thoughts on the theme of the paragone between the “sister arts”, an accomplished theory of exceptional coherence, built upon the first-hand experience of the sculptor’s craft, of which it is impossible to find a lucid formulation in the previously cited sources.