Many religious persons think that the highest reason for doing anything is that it is God’s Will: while to others ‘Self-realisation’ or ‘Self-development’, and to others, again, ‘Life according to nature’ appear the really ultimate ends. And it is not hard to understand why conceptions such as these are regarded as supplying deeper and more completely satisfying answers to the fundamental question of Ethics, than those before named: since they do not merely represent ‘what ought to be’, as such; they represent it in an apparently simple relation to what actually is. God, Nature, Self, are the fundamental facts of existence; the knowledge of what will accomplish God’s Will, what is, ‘according to Nature’, what will realise the true Self in each of us, would seem to solve the deepest problems of Metaphysics as well as of Ethics. But […] [t]he introduction of these notions into Ethics is liable to bring with it a fundamental confusion between “what is” and “what ought to be”, destructive of all clearness in ethical reasoning: and if this confusion is avoided, the strictly ethical import of such notions, when made explicit, appears always to lead us to one or other of the methods previously distinguished.

Henry Sidgwick, The Methods of Ethics, 7th ed., London, 1907, bk. 1, chap. 6, sect. 1