David Hume has been invoked by those who want to found morality on human nature as well as by their critics. He is credited with showing us the fallacy of moving from premises about what is the case to conclusions about what ought to be the case; and yet, just a few pages after the famous is-ought remarks in A Treatise of Human Nature, he embarks on his equally famous derivation of the obligations of justice from facts about the cooperative schemes accepted in human communities. Is he ambivalent on the relationship between facts about human nature and human evaluations? Does he contradict himself – and, if so, which part of his whole position is most valuable?
Between the famous is-ought passage and the famous account of convention and the obligations arising from established cooperative schemes once they are morally endorsed, Hume discusses the various meanings of the term “natural.” “Shou'd it be ask'd, Whether we ought to search for these principles [upon which all our notions of morals are founded] in nature or whether we must look for them in some other origin? I wou'd reply, that our answer to this question depends upon the definition of the word, Nature, than which there is none more ambiguous and equivocal.” (T. 473–74) The natural can be opposed to the miraculous, the unusual, or the artificial. It is the last contrast that Hume wants, for his contrast between the “artificial” culturally variant, convention-dependent obligations of justice and the more invariant “natural virtues,” and what he says about that contrast in this preparation for his account of the “artificial” virtues, makes it clear why he can later refer to justice as “natural” and to the general content of the rules of justice – that is, of basic human conventions of cooperation – as “Laws of Nature” (T. 484).