I wouldn't say it's an illusion; I would call it a perception, which I know exists because I do perceive it. In the same way I perceive external shapes and soun

I wouldn't say it's an illusion; I would call it a perception, which I know exists because I do perceive it. In the same way I perceive external shapes and sounds, I perceive thoughts about those perceptions and thoughts about my previous thoughts. That's not better than Descartes, but I think Descartes had this part essentialy right :-)