Principia Ethica

A treatise on the foundations of ethics that helped launch analytic moral philosophy. Moore argues that the central question of ethics is what 'good' means, and that good is a simple, indefinable, non-natural property, as yellow is a simple quality that cannot be explained to someone who has never seen it. Any attempt to define good in terms of a natural property, such as pleasure or what we desire, or a metaphysical one, commits what he calls the naturalistic fallacy. His open question argument supports this: for any proposed definition, it remains a sensible question whether that thing is in fact good. Moore then criticises the hedonism of Mill and Sidgwick and the metaphysical ethics of the idealists, and discusses how ethics relates to conduct, holding that right action is whatever produces the best outcome. The final chapter, 'The Ideal', applies his principle of organic unities, wholes whose value is not the sum of their parts, and names personal affection and aesthetic enjoyment as the greatest goods.

The naturalistic fallacy and the open question argument still frame debates in meta-ethics, and the book's final chapter influenced the Bloomsbury Group.