Weakness of the will remains a perplexing issue. Though philosophers have made substantial progress in homing in on what counts as a weak will, there is little agreement on whether weakness of the will is irrational, and if so, why. In this paper, we take an empirical approach towards the rationality of weakness of the will. After introducing the philosophical debate, we present the results of an empirical study that reveals that people take a “dual sensitivity”, as we shall put (...) it, towards assessing the rationality of weak-willed behavior. Put succinctly, intending X against your value judgements is assessed irrational; yet, in the same situation, intending X is assessed significantly less irrational if you judge X as something you ought to do. After discussing this result, we turn to the question of whether there is a plausible theory of rationality than can account for the dual sensitivity of the rationality assessments. We show that a success-based account can make sense of the dual sensitivity our empirical results reveal. (shrink)