I think he means the mind experiment from Descartes where he assumes everything around him has been created by some demon to fool him and in that case the only

I think he means the mind experiment from Descartes where he assumes everything around him has been created by some demon to fool him and in that case the only thing he can be certain about is that he is thinking (or you exist, I'm not sure). I'm also not an expert but that's what I understood from the original comment. Hope this helps