When should it be permissible to act against someone's expressed wishes in their best interests? In both political philosophy and legal practice, answers to this question often appeal to the concept of autonomy. Broadly, the idea is that if a person is sufficiently self-governing, then their wishes must prevail; but if they are not, then their wishes need not be respected when promoting whatever is good for them. This thesis analyses both philosophical models of autonomy and the practice of judges in England and Wales when implementing the Mental Capacity Act 2005. With regard to the philosophical models, it finds that, despite claims to the contrary, they do not offer a plausible way of assessing whether someone else is autonomous without appealing to values that are not the person's own. With regard to legal practice, it finds that, although judges speak about 'autonomy' in contradictory ways, a coherent account of when they will find that they must respect a person's expressed wishes can be constructed. This first stage of analysis makes a gulf between 'autonomy' in philosophy and law obvious. When philosophers talk about 'autonomy', they are largely concerned with the person's relationship to themselves. When judges talk about 'autonomy', they are largely concerned with the person's relationship to the world. 'Autonomy' in the philosophical sense cannot justify current practice because it does not deal with the same subject matter. Analysis of mental capacity cases does, however, allow the development of an alternative justification for actions against a person's expressed wishes. This justification lies in an evaluation of the entire situation, not of the person. It is not reducible to any model of autonomy, not even 'relational' models. Taken seriously, this justification requires a reorientation of the ethics of mental capacity law: away from overreliance on relatively few abstract 'principles' and towards articulating the difficulty and complexity of real situations. The thesis offers two papers towards the development of this latter mode.