Introduces the concept of mental capacity as a key meeting point between human freedom and mental disorder/disability. The emergence of a functional test of mental capacity, away from status and outcome tests, is discussed. An account is given of how the functional idea has been operationalised in mainly US–UK law and field tested in cases before a specialised court in England. This process is viewed as a classic one involving the public use of reason within a parliamentary democracy. Study of it has shown that an important romantic concern about the functional test (namely, that it overlooks the emotional or valuational aspects of human nature with an intellectual bias) are less compelling than was thought.