I suppose identity depends on memory. And if my memory is blotted out, then I wonder if I exist - I mean, if I am the same person. Of course, I don't have to solve that problem. It's up to God, if any.
The man who could withstand, with his fellow-men in single line, a charge of cavalry may lose all command of himself on the occurrence of a fire in his own house, because of some homely reminiscence unknown to the observing bystander.