Abstract
Twentieth century philosophy began with the rejection of naturalism. Many modern philosophers had assumed that their subject was continuous with the sciences, and that facts about human nature and other such information were relevant to the great questions of ethics, logic, and knowledge. Against this, Frege argued that “psychologism” in logic was a mistake. Logic, he said, is an autonomous subject with its own standards of truth and falsity, and those standards have nothing to do with how the mind works or with any other natural facts. Then, in the first important book of twentieth century ethics, Principia Ethica, G. E. Moore also identified naturalism as the fundamental philosophical mistake. Moore argued that equating goodness with any of the natural properties of things is “inconsistent with the possibility of any Ethics whatsoever”. Frege, Moore, and other like-minded thinkers inaugurated a period in which logic and language were the dominant philosophical subjects and confusing conceptual with factual issues was the greatest philosophical sin. During this period, philosophy was thought to be independent of the sciences. This may seem a strange notion, especially where ethics is concerned. One might expect moral philosophers to work in the context of information provided by psychology, which describes the nature of human thinking and motivation; sociology and anthropology, which describe the forms of human social life; history, which traces the development of moral beliefs and practices; and evolutionary biology, which tells us something about the nature and origins of human beings. But all these subjects were counted as irrelevant to the philosophical understanding of morality