His demolition crew included three classic arguments—concerning illusions, dreams, and demons—that cast doubt on our knowledge of the external world. These arguments weren’t entirely new. Illusions and dreams were standard fare for skeptics in ancient times such as Sextus Empiricus and the Roman orator Cicero, as well as for medieval thinkers such as the 5th-century North African saint Augustine and the Persian philosopher al-Ghazali.
