We have seen that, if human nature changes over time, and if human nature is essentially connected to morality through what is valuable about it, then morality changes over time too. But there is a further implication. If human nature changes over time, and reason is understood as the essential constituting feature of human nature, then reason changes over time too. The same conclusion follows from the views that morality changes over time, and that morality is constituted by reason. While a widespread belief in moral relativism demonstrates the acceptability of the idea that morality may change, the idea that the standards of reason may change is much less familiar to us. Plato's Form of the Good was presented by him as independent, eternal and consistent. We have throughout used this to characterize the apparent authority that human rights have for us. We have now seen arguments against their independence and eternity and in consequence against their universality, but consistency remains a dogma, one expressed in the claim that human rights – or, indeed, rights more generally – must be compossible. All rights are compossible just when all rights can be exercised at the same time, that is, that the action of one person exercising a right does not make impossible the exercise of rights by others.

If reason requires at some time that we believe something, and reason changes, then it may require at some later time that we believe something else. The point may be made clearer in the following way.