Full Idea
I am assured that as long as I think, I am myself. For I cannot think that I do not exist, nor exist so that I be not myself.
Gist of Idea
I cannot think my non-existence, nor exist without being myself
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1686.05.13)
Book Reference
Leibniz,Gottfried: 'The Leibniz-Arnauld Correspondence', ed/tr. Mason,HT/Parkinson,GHR [Manchester UP 1967], p.33
A Reaction
Elsewhere he qualifies the Cogito, but here he seems to straighforwardly endorse it.