The paper examines the way in which Salomon Maimon (1753-1800) combines Humean skepticism and Leibnizian rationalism to mount an innovative challenge to Kant. Maimon’s position can be described as an “apostate rationalism,” which holds that reason makes unavoidable demands on us that are nonetheless not satisfied in experience. An appreciation of Maimon’s arguments also sheds new and interesting light on the surprising role that this apostate rationalism plays as a component of Hume’s skeptical naturalism.