This article explores the development of law and policy relating to mental capacity law, situating this within the context of the binaries that have driven this development. Whilst the story of this historical development is well worn, considering it through this lens allows some of the previously hidden problematic consequences of these binaries to come to the fore in our debates. The article will expose these issues through considering the binary between capacity and incapacity, and the interlinked binaries of empowerment/protection and autonomy/paternalism underpinning policies and debate in this area. It will be shown that the struggles around the boundaries of this framework are becoming more pressing given the United Nations Convention on the Rights of Persons with Disabilities (UNCRPD), which presents a more fluid and potentially transformative framework for thinking about the legal subject in this context. There is here a danger, however, that we may end up falling too easily into seeing these issues through our current framework and in turn missing the opportunity to realise the potential of the UNCRPD. The article will disrupt and denaturalise the 'given-ness' of these conceptual boundaries, drawing on theoretical insights that seek to foreground relationality and the dynamic nature and processes of law. This opens the space in which to reimagine alternative trajectories for legal and ethical responses to take, and to provoke new conceptual pathways.