I gave my first university lecture in philosophy at the University of Ghana, Legon, when I was a freshly credentialed 21-year-old. My audience was a couple of hundred students gathered in a vast hall, with ceiling fans to move the hot and humid air. Above the murmur of the fans and the muttering of students, I tried to explain why Descartes thought the mere possibility that there was an Evil Demon deceiving their senses meant they couldn’t know for sure that I was really there. Ah, Cartesian skepticism! I remember diagraming the structure of the argument in huge chalk letters on an enormous blackboard.