Full Idea
Adequate necessary and sufficient conditions for identity which do not beg the question are very rare. …I don't know of such conditions for identity of material objects over time, or for people. Everyone knows what a problem this is.
Gist of Idea
No one seems to know the identity conditions for a material object (or for people) over time
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 1)
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.43
A Reaction
Typical of Kripke, who only seems to commit to conclusions suggested to him by his modal logic, and is baffled by almost everything else. I think one can at least attempt an essentialist approach to this problem.
Related Idea
Idea 16995 Given that a table is made of molecules, could it not be molecular and still be this table? [Kripke]