The United Nations Convention on the Rights of Persons with Disabilities (CRPD) presents in a tailored form the rights of such persons. Mental health disabilities are included. While the Convention is most welcome, it is hugely challenging when it comes to involuntary treatment. Important authorities have interpreted it as excluding all forms of ‘substitute decision-making’. The Convention demands ‘respect for the rights, will and preferences’ of persons with disabilities. This chapter examines the meaning of ‘will’ and of ‘preference’. A problem arises when a person’s currently expressed ‘preference’ (or desire or wish) diverges from the person’s ‘will’ (taken to mean a person’s relatively enduring and deeply held value commitments). Both cannot be respected at the same time. Which should have precedence? The method of ‘interpretation’ discussed in Chapter 7 allows such a determination to be made, and aligns the ‘fusion law’ proposal with the objectives of the Convention.