In this contribution, I offer a study of what I take to be the core argument of Augustine’s De Immortalitate Animae, a series of notes that, once reworked into dialogue form, were designed to form book III of his Soliloquies. Taking his starting-point from the premise that a structured body of truths (disciplina), e.g. geometry or arithmetic, always exists, and that it must inseparably exist in a subject, i.e. the soul, Augustine claims to deduce that the soul is immortal. This argument contrasts interestingly with the arguments from recollection in Plato’s Phaedo and Meno, which begin from the premise that the soul’s innate knowledge could not have been acquired in a person’s lifetime. Innate knowledge, however, is no guarantee of the soul’s eternal pre- or post-existence, since the soul could have come to be in time with its knowledge already present. Augustine’s method of argument, which makes the soul’s immortality depend on the eternity of truth, avoids this objection, only to face other serious difficulties of which he himself was well aware. I will discuss a number of these problems, before finally considering whether the concept of ‘reason’ (ratio) in chapters 10-11 of his embryonic treatise can establish a firm link between truth and immortality.