Full Idea
Since therefore I must exist in order to be mistaken, then even if I am mistaken, there can be no doubt that I am not mistaken in my knowledge that I exist…. I know that I exist, and I also know that I know.
Gist of Idea
I must exist in order to be mistaken, so that even if I am mistaken, I can't be wrong about my own existence
Source
Augustine (City of God [c.427], Ch.XI.26)
Book Reference
Scruton,Roger: 'Modern Philosophy: introduction and survey' [Sinclair-Stevenson 1994], p.460
A Reaction
Fine, but the main problem is his over-confidence about a stable personal identity that does the thinking.