In his Commentary on Plato’s Timaeus, the fourth-century exegete Calcidius makes ample use of musical theory and its kinship with the related sciences arithmetic, geometry and astronomy. Musical theory impacts upon the metaphysical, physical and ethical aspects in his account of the composition of the cosmic soul. Calcidius draws upon the soul’s musical make-up to show how it translates into the arrangement of the heavenly bodies, the relationships between the immortal and mortal creatures in the cosmos, and the tripartite human soul. Emphasizing the overall significance of harmonics for his exegesis, Calcidius, in fact, likens the creator god to a musician who composed the All as a well-tuned symphony. I shall discuss these aspects of his exegesis by placing them, initially, into the context of his sources, thereafter focusing on more idiosyncratic aspects: Calcidius’ referencing of musical composition for the question of the soul’s (un-)createdness, the relationship he establishes between harmonics and his demonology, and his view on human psychological conditions such as anger and passion. These conditions, according to Calcidius, are not merely a consequence of the human soul’s association with the body, but psychological manifestations of its natural make-up, which is determined by numeric-musical proportions.