In a famous passage in the Treatise, Hume tells us that no "ought" sentence can be derived from premises which are purely factual. 1 It is usual to extend this rule to forbid the derivation of any evaluative sentence, "Xis right," "Xis obligatory," etc., as well as "X ought to be done," from factual premises. I shall refer to this strong rule as Hume's Rule. This rule is of the greatest importance in ethics because it seems to guarantee the autonomy of values.
Proponents of autonomy hold that an agent is free to adopt values in a way in which he is not free to adopt factual claims. A man who holds a particular scientific theory can be presented with objective evidence which he must recognise as counting for or against this theory; refusal of this recognition amounting to unreasonableness or irrationality. Factual claims must always stand in jeopardy from the discovery of counter-evidence. Once Hume's Rule is accepted, evaluations are insulated from fact. Whatever facts are discovered and whatever factual sentences seem acceptable, these have no consequences of an evaluative nature. Facts can never force us to a particular evaluation, nor prevent us from holding the values which we do hold. If Hume's Rule is correct, then in Hate's words? 9 it follows that we are free to form our own moral opinions in a much stronger sense than we are free to form our own opinions as to what the facts are.
Popper adopts the same positions when he argues that the duality of facts and standards means that: 3 Neither nature nor history can tell us what we ought to do. Facts, whether those of nature or those of history, cannot make the decision for us, they cannot determine the ends we are going to choose. If Hume's Rule is correct, then it seems clear that facts and values are autonomous and that we enjoy a special freedom in our evaluations. The reverse is, however, not so clear. As we shall see, it may be possible for Hume's Rule to be incorrect and yet for the doctrine of autonomy to be true. The correctness of Hume's Rule is a sufficient, but perhaps not a necessary condition for the autonomy of values.