Full Idea
To know that one exists is not to know anything about oneself any more than knowing that 'this' exists is knowing anything about 'this'.
Gist of Idea
Knowing I exist reveals nothing at all about my nature
Source
A.J. Ayer (The Problem of Knowledge [1956], 2.iii)
Book Reference
Ayer,A.J.: 'The Problem of Knowledge' [Penguin 1966], p.52
A Reaction
Descartes proceeds to define himself as a 'thinking thing', inferring that thinking is his essence. Ayer casts nice doubt on that.