Full Idea
If any impression gives rise to the idea of self, that impression must continue invariably the same, through the whole course of our lives; since self is supposed to exist after that manner. But there is no impression constant and invariable.
Gist of Idea
A continuous lifelong self must be justified by a single sustained impression, which we don't have
Source
David Hume (Treatise of Human Nature [1739], I.IV.6)
Book Reference
Hume,David: 'A Treatise of Human Nature', ed/tr. Selby-Bigge/Nidditch [OUP 1978], p.251
A Reaction
This is a rather dogmatic application of the requirement that all knowledge must be founded in experience. It fails to recognise that knowledge of the thing having the experiences is a rather special case. We must ask for the best explanation.