At the end of the nineteenth century, the Harvard psychologist William James laid down a cornerstone of modern self theory. In his 1890 Principles of Psychology, James distinguished between the self as knower (the I) and the self as object known (the Me, or self-concept). This formulation offered a language for talking about matters that had been obscured by reifications such as psyche, mind, soul, spirit, and ego. Following James, the self could be seen as both a process—acts of perception and knowing—and the outcome of that process—knowledge about the knower. James's distinction remains basic to self theory today.

The origins of self theory lie in human prehistory. As our hominid ancestors sought to explain the world around them, they likewise struggled to explain themselves. The ...