As a number of critics have noted, Anthony Trollope tended to reuse a particular version of the marriage plot. What Victoria Glendinning calls his ‘Ur-story’ is a version of the romantic triangle: protagonists, usually though not always male, commit to marrying one character, but then find themselves drawn to a second. Trollope varies the specific plot dynamics; sometimes the protagonist will succeed in returning to the first character, sometimes he will abandon his previous commitment, and often complications produce other outcomes. This aesthetic fact leads to a recurring consideration of a particular issue in philosophical psychology; moral philosophers have long been interested in situations where moral agents know what they ought to do but do not do it.

At the most general level, the theoretical problem involved in such states, which philosophers describe as instances of ‘weakness of the will’ or ‘akrasia’, is a question in the logic of moral psychology: how can agents will something and not will it at the same time? On what one might call the simple philosophical model, ordinary action proceeds by an agent judging that a given action is worth performing; this decision constitutes an intention and produces an action. Hence it is not immediately clear how akratic action – which occurs somehow against an agent's judgement – is possible. Since agents frequently do seem to act against their better judgements, the simple model of intentional action must be inadequate in some way, yet this model is so intuitive that philosophers have often thought that akrasia proper in fact rarely occurs, and that most cases of weakness of will involve a sort of confusion about what one's judgements actually are. For example, the first account of the problem, Plato's discussion in the Protagoras, claims axiomatically that ‘to make for what one believes to be evil’ is not in ‘human nature’. When agents appear to do so, they are really confused about what course of action is best; as Amelie Rorty explains, Plato's ‘account of akrasia explains away counterexamples by re-describing them as cases of deception of some sort’.