Full Idea
If a man were made young again, and learned everything anew - would that make him a different man? So it is not memory that makes the very same man.
Gist of Idea
Memory doesn't make identity; a man who relearned everything would still be the same man
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.01)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.114
A Reaction
Leibniz takes this as a foregone conclusion. If you flipped to a possible world where someone you know well, as a physical being, has been brought up entirely differently (new language, culture, ethics etc), is it really the same person?