Full Idea
Not only is it immediately evident to me that I think, but it is just as evident that I think various thoughts: at one time I think about A and at another about B and so on. Thus the Cartesian principle is sound, but it is not the only one of its kind.
Gist of Idea
I know more than I think, since I know I think A then B then C
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 4.02)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.367
A Reaction
I don't suppose that Descartes would object to this, but he was aware that there didn't seem to be any actual introspective experience that united the various thoughts into a single thinker. Only logical connections between the thoughts does that.