Full Idea
In the Cogito the work is all done by the demonstrative word 'I', because it is a precondition of the use of such a word that the thing to which it points has to exist.
Gist of Idea
It is a precondition of the use of the word 'I' that I exist
Source
comment on René Descartes (Meditations [1641], §2.26) by A.J. Ayer - The Problem of Knowledge Ch 2 (iii)
Book Reference
Ayer,A.J.: 'The Problem of Knowledge' [Penguin 1966], p.50