In medical ethics, there is a well-established debate about the authority of advance directives over people living with dementia, a dispute often cast as a clash between two principles: respecting autonomy and beneficence toward patients. This chapter, in highlighting underexplored issues of power and social status, argues that there need be only one principle in substitute decision-making: determining authenticity. This principle favours a substituted judgment standard in all cases and instructs decision-makers to determine what the patient would authentically prefer to happen – based not merely on the patient’s decisions but also on their present settled dispositions. Adhering to this principle entails that, in a significant range of cases, an advance directive can (and indeed ought to) be overruled.