 The first reaction to non-naturalism, other than simple acceptance, came from philosophers who endorsed the autonomy of ethics but, sometimes under the influence of logical positivism, rejected its moral realism, holding instead that there are no facts other than natural ones and no modes of knowing other than the empirical and the strictly logical. They therefore developed various versions of non-cognitivism, which hold that moral judgements are not true or false but express attitudes, as in emotivism (Ayer 1936; Stevenson 1944), or issue something like imperatives, as in prescriptivism (Hare 1952; Hare 1963). Unlike the subjective naturalism Moore criticized in Ethics, these views allow moral disagreement, since attitudes and imperatives can oppose each other, for example positive versus negative. They also, their later proponents held, give a better explanation of the open-question argument, since they find a distinctive emotive or action-guiding force in moral concepts and judgements that is not present in non-moral ones; that is why, it was said, the moral is neither reducible to nor derivable from the non-moral. Non-cognitivism can also explain, some said, why morality matters to us as it does. Non-naturalism implies that moral judgements concern a mysterious type of property, but why should facts about that property be important to us or influence our behavior? If moral judgements express deep-seated attitudes, however, the question answers itself.