What did Moore mean by the "Naturalistic Fallacy"? What is "naturalistic" about it? Is it really a fallacy? Has either G J Warnock or Philippa Foot succeeded in establishing the legitimacy of ethical naturalism?

"If I am asked, "What is good?" my answer is that good is good and that is the end of the matter. Or if I'm asked, "How is good to be defined?" my answer is that it cannot be defined, and that is all I have to say about it".
G.E. Moore, "Principia Ethica"

Indeed it does appear that this is all Moore has to say about it. The absence of any acceptable "proof" that his position is more tenable than that of the ethical naturalists or metaphysicians makes it necessary for him to make this point so forcibly. The quotation above is the meta-ethical equivalent of the maxim of Bishop Butler's on the title page of "Principia Ethica", "Everything is what it is, and not another thing."; We are here concerned with meta-ethics, that is, the meaning of ethical terms, rather than with ethics proper which, as that branch of philosophy which deals with human behaviour, might be described as the practical application of philosophical ideas. It was only within the field of meta-ethics that G.E.Moore w...