Was Descartes a Liar? Diagonal Doubt Defended

Abstract

Arguably, among the most discussed philosophical issues in the history of philosophy have been the Liar paradox and Descartes's cogito argument. That these should turn out to be variants of a common underlying puzzle is admittedly a surprising and even eccentric claim at first sight, but it was the burden of my paper 'Descartes's Diagonal Deduction' [1983] to argue at length for just this view (for further details, see Slezak [1985]). If nothing else, the thesis can claim to be a bold conjecture in the Popperian sense of sticking its neck out. To my knowledge, no one had observed this connection before, and this would be surprising if my claim were correct. On its own, of course, such 'boldness' is not necessarily a virtue for it is an attribute shared by all insane and unorthodox theories. Merely remaining unrefuted is no virtue either, for this might happen by default, as it were, just through neglect. Like Norman Bloom's proof of God's existence from the Manhattan telephone directory (Sagan [1979]), one's theory might remain unrefuted simply because no one bothers to take up the challenge. Accordingly, I am grateful to Roy Sorensen [1986] for his recent challenge to my claims concerning Descartes's argument, but I believe these claims survive unscathed, as I should like to show here.