Full Idea
The simplicity of my self is not inferred from the proposition "I think", but rather the former lies in every thought. 'I am simple' is an immediate apperception, just as the Cogito is tautological, since 'cogito' immediately asserts the reality.
Gist of Idea
My self is not an inference from 'I think', but a presupposition of it
Source
comment on René Descartes (Meditations [1641], §2.26) by Immanuel Kant - Critique of Pure Reason A355
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.419
A Reaction
This is why Kant thinks the self is the result of a transcendental deduction, rather than of a direct observation of the self-evident. Personally I side with Descartes. I do not 'observe' my self, but I am acutely aware of its presence and actions.