Principia Ethica is a book written in 1903 by British philosopher, G. E. Moore. Moore questions a fundamental pillar of ethics, specifically what the definition of "good" is. He concludes that "good" is indefinable because any attempts to do so commit the naturalistic fallacy. Principia Ethica was influential, with Moore's arguments being considered ground-breaking advances in the field of moral philosophy. Source: Wikipedia (en)