It is nearly universally thought that the kind of decision‐making competence that gives one a strong prima facie right to make one's own medical decisions essentially involves having an ability (or abilities) of some sort, or having a certain level or degree of ability (or abilities). When put under philosophical scrutiny, however, this kind of theory does not hold up. I will argue that being competent does not essentially involve abilities, and I will propose and defend a theory of decision‐making competence according to which one is competent only if one possesses a certain kind of rationality in making treatment decisions.