Full Idea
To say 'I think therefore I am' is not really to prove existence from thought, since 'to think' and 'to be thinking' are one and the same, and to say 'I am thinking' [je suis pensant] is already to say 'I am' [je suis].
Gist of Idea
The Cogito doesn't prove existence, because 'I am thinking' already includes 'I am'
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 4.07)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.411
A Reaction
This is the objection which was offered by A.J. Ayer, and I take it to the one of the two principle objections to the Cogito (i.e. that it may be a tautology), along with the objection about the assumption of the continuity of the same thinker.