The 1962 publication of J. H. Waszink's edition of Calcidius' commentary on Plato's Timaeus focussed attention on the question of Calcidius' source for a group of chapters where he presents an interpretation of Plato's account of the creation of soul. I discuss three attempts to answer this question: that of Waszink himself, who argues that the source is Porphyry who was here influenced by the Neopythagorean/Platonist Numenius, that of J. M. Van Winden, who claims Numenius as the direct source, and that of Werner Deuse, who offers reasons for excluding Numenius as either directly or indirectly responsible. I show the weakness of Deuse's critique, but then argue that neither Waszink nor Van Winden has given sufficient consideration to the possibility that Plotinus, either alone or through Porphyry as intermediary, is behind Calcidius' interpretation, where the doctrines of the unity of the soul and of a higher soul that does not descend, both characteristic of Plotinus' psychology, are prominent. However, there is an important but uniformly overlooked feature of this exegesis that rules out both Plotinus and Porphyry as possible sources. This leaves Numenius as the only serious candidate. If, then, the origin of the doctrines of the unity of the soul and of the undescended higher soul can be traced to the work of Numenius, there is the strong possibility that in his formulation of these same doctrines Plotinus was indebted to Numenius.