 Personal identity is perhaps the most extensively discussed special case of identity. What is it for a person existing at one time to be identical to a person existing at another? This question was first clearly posed by John Locke in his celebrated discussion of personal identity in the Essay (Locke 1975). Locke distinguishes between being the same man and being the same person. To be the same man is to be the same member of the species human being. For Locke what is crucial is that having the same consciousness is not sufficient for being the same man, but is sufficient for being the same person.