This paper argues that Augustine did not take the proof inSoliloquia2.22-4, which centers on the Aristotelian notion of ‘being in a subject’, from a single source but constructed it in a deliberately imperfect manner from several passages from Porphyry’s works on Aristotle’sCategoriesin order to supplement it with further arguments in Book Three. InDe immortalitate animaeAugustine explicitly discloses the weaknesses of the proof and repairs them by means of a Neoplatonic notion of causality.