Full Idea
By taking Cogito Ergo Sum as the only certainty, and by his provisionally regarding the existence of the world as problematical, the essential starting point of all philosophy was found, and its true focus in the subjective, the individual consciousness.
Clarification
The 'Cogito' is Descartes' famous "I think therefore I am"
Gist of Idea
Descartes found the true beginning of philosophy with the Cogito, in the consciousness of the individual
Source
Arthur Schopenhauer (The World as Will and Idea [1819], I Supp)
Book Reference
Schopenhauer,Arthur: 'The World as Will and Idea', ed/tr. Berman,Jill and David [Everyman 1995], p.12
A Reaction
Some people think this was a disaster, not a triumph. Descartes could have doubted himself and accepted the world as his starting point.