Full Idea
Every idea is derived from preceding impressions; and we have no impression of self or substance, as something simple and individual. We have, therefore, no idea of them in that sense.
Gist of Idea
We have no impression of the self, and we therefore have no idea of it
Source
David Hume (Treatise of Human Nature, + Appendix [1740], Appendix)
Book Reference
Hume,David: 'A Treatise of Human Nature', ed/tr. Selby-Bigge/Nidditch [OUP 1978], p.633
A Reaction
This spells out with beautiful simplicity how his empiricist assumptions lead him to this sceptical view. No logical positivist could reject this thought. Personally I favour empiricism with added inference to the best explanation.