Full Idea
If I know I doubt, I have as certain a perception of the existence of the thing doubting, as of that thought which I call doubt.
Gist of Idea
I am as certain of the thing doubting, as I am of the doubt
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.09.03)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.618
A Reaction
The challenge to this Lockean assertion of the Cogito is what he means by a 'thing', and what grounds he has for asserting the existence of the 'thing', as opposed to some vague assertion about whatever makes doubting possible.