An interest in human nature has long been a motive driving philosophical inquiry. In the ancient world it was associated most closely with the name of Socrates and with his claim that the most valuable knowledge is self-knowledge. In the modern period a distinctive kind of philosophical thought got under way with Descartes's attempt to show that rigorous scientific knowledge must be grounded in a new kind of self-knowledge. In the following century, David Hume published A Treatise of Human Nature, in which he dealt with all the major issues of philosophical inquiry as facets of that most general topic. Most notably, perhaps, Immanuel Kant argued that the domain of philosophy was defined by three questions – What can I know?, What ought I to do?, and What may I hope? – and that these questions are facets of the more general question, What is man?

An interest in human nature can take many forms, and the questions in which it finds expression can range from such matters as the character of human motivation (Is altruistic conduct really possible?) to the prospects for human happiness (Can a man really be called happy before his life is complete?). Philosophers have asked questions like these, but they have been more interested in what differentiates human beings from other living things and generally from the natural world in which they live. The answer they have most often given to such questions is that the distinctive features of human nature have to do with the mental functioning of human beings – more specifically, with their intellectual and moral powers.