Principia Ethica is G. E. Moore's outstanding treatise of moral philosophy, wherein the philosopher posits that what is 'good' is indefinable, while demonstrating the fallacy of naturalism. Upon its original publication in 1903, this text received strong reception within academia for its convincing and clearly stated views. Moore firmly believed that moral values could not be based upon a simple knowledge, but instead from rigorous and intuitive experience of life's pleasures - beholding beauty, enjoying friendships, and taking pleasure in activities. In this book, Moore convinces readers of his argument with a series of expositions which are today held as advancing the field of moral philosophy. Influential in 20th century philosophy, this work was praised by Moore's contemporary Bertrand Russell and others in the scholarly community. For the student or enthusiast of modern moral philosophy, the Principia Ethica is vital reading. This edition presents the original text in full, with no abridgement made.