To have a sense of personal existence distinct from others may be unique to human beings, and in large measure due to the learning of language. Jung and Neumann’s studies of the historical development of identity suggest, in an evolutionary sense, that having an ‘I’ is still a very newly acquired function. This makes it vulnerable. It is also noticeably something which develops during childhood and reaches different levels of maturity during adulthood. Although it is our central experience, it remains an enigma – a will o’ the wisp which loses itself in dreams and sleep, yet is so dominant and sure in waking.