This chapter focuses on Proclus’ use of a theological notion of harmony, which is designed to reveal the essence, intelligible relations, and causality of the soul by taking its harmonic structure as a starting point. The fact that the soul is made of specific means and proportions paves the way to the claim that the soul’s essence consists of a logos. This represents neither just an exegetical remark related to Plato’s divisio animae nor the mere use of an image: Proclus regards Plato’s account of the soul’s harmonic structure as a specific key to access theology. By analysing the harmonic component within Proclus’ iconic theology, a clear analysis of both the “theological” implications of Proclus’ study of the harmonic structure of the Platonic world-soul and of the metaphysical-theological function of the ambivalent notion of logos emerges.