Descartes believed that there was a logical path from doubting the existence of the body to affirming mind-body dualism. In the 20th and 21st centuries, a critique of Cartesian reasoning first made by Arnauld in 1641 has been revived and widely accepted. Several writers including Paul Churchland and Gary Hatfield make the case that the argument for dualism commits the masked man fallacy; that the Cartesian argument relies on mere ignorance of the body to reach its conclusion. In this paper, I show that the argument from Cartesian doubt to mind-body dualism does not depend on mere ignorance. It depends on reliable knowledge about what can and can not be known. Descartes’ method of doubt leads to the conclusion that the body can never under any circumstance be known as the mind is known. The argument for dualism rests on that knowledge, not on ignorance. This paper reveals a viable Cartesian argument for mind-body dualism and explicates the missteps of Descartes’ contemporaneous and present-day critics.