Kant felt that Descartes wrongly described the certainty of self knowledge, I cannot extend my sceptisism into the subjective sphere, so I can be sure of my present mental states, but I cannot be sure of what I am or of whether or not there is an 'I' to whom these states belong.
Kant argued that doubt shows there is a point of view and that the transcendental unity of apperception provides the minimal description of our point of view, I can know one thing - that there is a unity of consciousness. The transcendental deduction shows that the truth of transcedence is presupposed in the existence of experience.
So thought presupposes transcendental unity, not that I am.
I think
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Edit:- I should add that as with everything Kantian I'm not sure I understand what he was trying to say - but this is what I took from it.