In 419, or a little later, Julian attempted to pin a charge of Manichaeanism on Augustine, claiming, among other things, that Augustine's account of sexuality in marriage implied that the marriage act is vicious in itself. For Augustine had insisted from as early as The Desserts of Sinners (1.29.57) that the ‘disobedient members’ indicate original sin and the birth of children ‘in concupiscence’ (2.4.4).

The ensuing argument, still unfinished when Augustine's death prevented the completion of the last work Against Julian, was partly provoked by Augustine's use of the word concupiscentia itself. As we have seen, what he meant to represent by this term, at least in the phrase concupiscentia carnis, was a generalized weakness of the ‘flesh’, to which we can improperly assent. But ‘misunderstanding’ arose from the fact that, though concupiscentia is not limited to sexual desire, autonomous sexual arousal, being its most obvious visible effect, conveniently served as a symbolic representation of the phenomenon as a whole.

Augustine's assessment of the origin of this general weakness, which he saw as a defect, not a sin, and which in its sexual form makes males look like animals, is that it is acquired as a penal result of the fall. According to Julian, however, Augustine's ‘real’ view is that sexual desire is sinful in itself, not merely that it is vitiated by the sinfulness of Adam. Against this rendering of Augustine's position, Julian offered his own alternative, that libido as presently experienced is to be described as a morally neutral ‘natural appetite’ (Marriage and Concupiscence 2.7.17, apparently only in men (Against Julian 5.5.23)), or as ‘vigour of the members’ (2.35.59).