Given Hume's approach, we cannot be directly motivated by rights as independently real, nor by reason similarly understood. We can be motivated only by desire. Moreover we are not to understand the content of any moral claim, including a claim to a moral right, as involving some reference to an independent moral reality. And yet the human capacity for rationality – that essential feature which we supposedly all have to be reasonable – is imagined by Plato, Hobbes and Locke to consist at least partly in a respect for standards of reason that are in some way external to us, however they are known. It is this that is often thought to make us characteristically human and distinct from other animals. Even the materialist Hobbes sees reason in this way. Reason, furthermore, in some way necessarily constitutes or expresses or is involved in – even if only minimally – morality. We face here a polarization of alternatives: the conflict between empiricism and rationalism. It is Kant who tries to resolve that conflict, and he does so through a complex presentation of a theory of our human nature and our relationship to reality.

At this point we need to understand that the theory of motivation effectively – so far – has two central functions in moral theory and, within that, for the theory of rights. First, a theory of motivation acts as a constraint for any acceptable theory of morality in the following way: a theory of morality must not be conceived to make demands on us that we cannot understand or know about or be motivated to follow.