Full Idea
The fact that I am a subject ..does not signify that as object I am a self-subsisting being or substance; the latter goes too far, and hence demands data that are not encountered at all in thinking.
Gist of Idea
The fact that I am a subject is not enough evidence to show that I am a substantial object
Source
comment on René Descartes (Meditations [1641], §2.26) by Immanuel Kant - Critique of Pure Reason B407
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.446
A Reaction
This is a key problem with the Cogito - that so little can be said about the 'I' of which the existence has been proved that it is not clear that anything has been proved at all - certainly not that there is a continuous and stable Ego.