Principia amounts to G. E. Moore’s best attempt at developing what he sometimes referred to as a “scientific ethics.” The general met ethical questions that interested Moore most in Principia Ethica – together with his references to ethics as a “science” that is somehow analogous to physics and chemistry – appears to give credence to an objection by P. H. Nowell-Smith, according to which, in the early twentieth century, Moore and other classical on-naturalists had changed the subject matter of ethics. Moore held that investigation in either general or applied normative ethics requires prior knowledge of some major issues of philosophical ethics, the branch of ethics now known as ‘metaethics.’ In Moore’s time, just as today, major challenges to Autonomy stem from psychology and the theory of evolution. Independence and Priority are theses about the relation of metaethics with other branches of ethics that have Principia Ethica as their main source.