I remember spending hours agonizing over this idea. How do I know if my eyes and brain are working correctly? Any thought process that might lead me to a conclusion would be taking place in my brain. The same brain that I want to prove works correctly. The best I could come up with was that if my brain works correctly, I stand to gain by operating under the assumption that it does, and I stand to lose by operating under the assumption that it doesn't. If my brain does not work correctly, then I have no basis for any conclusion so it makes no difference what my operating assumptions are.