George Edward Moore's Principia Ethica was published in 1903 and is widely recognised as the definitive starting point for modern ethical theory. Moore shifts the focus of philosophy by applying rigorous logic to ethics, defining it as an inquiry into what is genuinely good. He challenges common-sense assumptions and demonstrates how false premises about goodness lead to incorrect ethical conclusions. This Briefly guide is designed to make Moore's foundational thoughts accessible to you, providing a clear path through his critique of naturalism and his influence on thinkers like Russell and Wittgenstein. It is an essential companion for any student navigating the development of 20th-century ethics.