Is there a human nature? Can knowledge of it help us live better lives? This book synthesises ancient and modern philosophical ideas and draws on scientific research to answer yes to both these questions. It develops an innovative normative theory on the basis of commonsensical, naturalistic, premisses; and it defends an Aristotelian normative theory -- whereby we should understand human goods as realisations or perfections of human nature -- against both traditional and emerging challenges to perfectionist ethics, including evolutionary biology and transhumanism. The result is a ground-breaking theory of 'natural perfectionism', which both returns perfectionistic ethics to its Aristotelian roots and shows how this is compatible with evolutionary biology and cognitive science. At a time when the very idea of human nature is viewed as something that can be readily transcended, this work recalls us to a realistic, sober and better-founded vision of it.