 G.E. Moore reacted against views that provided naturalistic definitions of moral concepts, especially that of goodness (see entries on  naturalism,  and  moral naturalism).  One might question whether his targets actually held the view he criticized, but let’s set such worries aside and assume that as the 19th-century drew to a close, philosophical ethics was dominated by hedonists who defined “good” in terms of pleasure, evolutionary ethicists who defined “good” as the most evolved, utilitarians who defined “right” as what produces the most utility, and other naturalists. According to such views, fundamental moral principles are analytic, and hence knowable a priori. Since the fundamental principles defined moral concepts in natural terms, one can infer whether some specific thing is good or action right from the principle conjoined with appropriate propositions about the natural properties of the thing or action, the latter being knowable empirically. For example, one could infer that listening to music is good from the analysis of “good” as conducive to pleasure and the fact that listening to music is often pleasant. In presenting Moore’s response to such naturalistic definitions, we will focus on the position he took in Principia Ethica (1903 [2003]) (although his views changed significantly over time).