Full Idea
The sentence "I think" contains a series of unprovable assertions; for example, it is I who think, that it must be something at all which thinks, that thinking is by an entity thought of as a cause, that an 'I' exists, and that I know what thinking is.
Gist of Idea
'I think' assumes I exist, that thinking is known and caused, and that I am doing it
Source
comment on René Descartes (Meditations [1641], §2.26) by Friedrich Nietzsche - Beyond Good and Evil §16
Book Reference
Nietzsche,Friedrich: 'Beyond Good and Evil', ed/tr. Hollingdale,R.J. [Penguin 1973], p.28