Full Idea
The full conditions of personal identity cannot be extracted from the concept of a person at all: they cannot be arrived at a priori.
Gist of Idea
Personal identity cannot be fully known a priori
Source
Thomas Nagel (The View from Nowhere [1986], III.2)
Book Reference
Nagel,Thomas: 'The View from Nowhere' [OUP 1989], p.35
A Reaction
However, if you turn to experience to get the hang of what a person is, it is virtually impossible to disentangle the essentials from the accidental features of being a person. How essential are memories or reasoning or hopes or understandings or plans?