  Moore himself later abandoned this definition of the distinction between natural and non-natural properties, and described his earlier account of a natural property as “utterly silly and preposterous” (1942, 582). In the Preface to the second edition of Principia Moore offers an alternative definition that is suggested in chapter two of Principia. According to this definition, a natural property is one “with which it is the business of the natural sciences or of Psychology to deal” (13). Since the term to be defined (‘natural’) appears in the definition, this definition may not seem very informative. But we can replace the term ‘natural sciences’ with ‘empirical sciences’ (understood to include psychology and sociology) to get a useful and workable epistemological definition of a natural property.  On this account, then, natural facts can be known by purely empirical means, whereas non-natural moral facts cannot be known in this way.  Such facts involve an essentially a priori element.