Full Idea
The fact that a thought occurs at a given moment does not entail that any other thought has occurred at any other moment, still less that there has occurred a series of thoughts sufficient to constitute a single self.
Gist of Idea
A thought doesn't imply other thoughts, or enough thoughts to make up a self
Source
comment on René Descartes (Meditations [1641], §2.26) by A.J. Ayer - Language,Truth and Logic Ch.2
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.63
A Reaction
This seems to be the main objection to the Cogito. It doesn't refute it, but simply recommends cautious restraint in what is being claimed as its conclusion. I can't make much sense of a thought which has no thinker at all.