Over the last fifty years, scholars in biology, psychology, anthropology, and cognate fields have substantially enriched traditional philosophical theories about who we are and where we come from. The assumption of a shared human nature lies at the core of some of the most pressing socio-political issues of our time. From race to sex and gender, from medical therapy to disability, from biotechnological enhancement to transhumanism, all these timely debates presuppose a robust notion of human nature. Nevertheless, the riddle of human nature remains frustratingly elusive. Why? Marco J. Nathan here provides an accessible, detailed, and up-to-date overview of cutting-edge empirical research on human nature, including evolutionary psychology, critiques of essentialism, innateness, and genetic determinism, addressing the question of why these fields have failed to provide a full-blown theory of human nature. Nathan's answer is that our nature is not the kind of notion that is susceptible to explanation. Human nature rather plays a crucial role as an epistemological indicator, a pivotal concept that sets out the agenda for much social, political, and normative discourse. Nevertheless, science cannot adequately grasp it without dissolving it in the process.