In this classic work, best-selling author Harry Frankfurt provides a compelling analysis of the question that not only lies at the heart of Descartes's Meditations, but also constitutes the central preoccupation of modern philosophy: on what basis can reason claim to provide any justification for the truth of our beliefs? Demons, Dreamers, and Madmen provides an ingenious account of Descartes's defense of reason against his own famously skeptical doubts that he might be a madman, dreaming, or, worse yet, deceived by an evil demon into believing falsely.

Frankfurt's...