Full Idea
If Socrates says he doubts everything, it necessarily follows that he at least understands that he doubts, and that he knows that something can be true or false: for these are notions that necessarily accompany doubt.
Gist of Idea
When Socrates doubts, he know he doubts, and that truth is possible
Source
René Descartes (Rules for the Direction of the Mind [1628], 12)
Book Reference
Descartes,René: 'Rules for the Direction of the Mind' [Newcomb Library 2023], p.40
A Reaction
An early commitment to the Cogito. But note that the inescapable commitment is not just to his existence, but also to his own reasoning, and his own commitment, and to the possibility of truth. Many, many things are undeniable.