Immanuel Kant’s notion of weakness or frailty warrants more attention, for it reveals much about his theory of motivation and general metaphysics of mind. As the first and least severe of the three grades of evil, frailty captures those cases where an agent fails to act on their avowed recognition that the moral law is the only legitimate determining ground of the will. The possibility of such cases raises many important questions that have yet to be settled by interpreters. Most importantly, should we account for the failures of weakness by appealing to the activity of reason or sensibility? I will discuss this question in light of a tendency to adopt an overly dualistic reading of Kant’s moral psychology. Focusing on Kant’s remarks on weakness from the Religion and the Metaphysics of Morals, I argue that we should understand weakness as arising from the unique difficulties of sense-dependent judgment, rather than from self-deception, flagging commitment, or overwhelming desire. The resulting account offers a unified moral psychology capable of accommodating the many features of weakness that are difficult to reconcile on other readings.