 By contrast with an ‘objectivist’ interpretation of the BIP what is best for a child can be understood in terms of a child’s hypothetical choices. Strictly speaking a hypothetical choice interpretation of the BIP amounts to a distinct principle. It is one Buchanan and Brock define as that of the ‘substituted judgement’—‘acting according to what the incompetent individual, if competent, would choose’ (Buchanan and Brock 1989, 10). However it is natural to think that what is best for someone is what they themselves would choose if fully informed and deliberating fully rationally. Thus a striking and influential thought in this context is that we choose what is best for the child if we choose for the child as the child would choose for herself if the child were adult. For instance John Rawls thinks the following formulation defines the acceptable paternalism of a guardian’s treatment of his child: ‘We must choose for others as we have reason to believe they would choose for themselves if they were at the age of reason and deciding rationally’ (Rawls 1999, 183). This apparently simple formulation is in fact susceptible of three quite different interpretations, each of which brings with it its own problems. In each case we are seeking to specify the adult person who chooses for the child.