 In spite of a pre-reflective disposition toward dualism, an explicit argument for an agnostic attitude with respect to the ultimate constitution of reality is also characteristic of John Locke (1632–1704). Already in The Epistle to the Reader of An Essay concerning Human Understanding (1690) he denounces rationalist metaphysics as a “Sanctuary of Vanity and Ignorance” and declares in the first book of his Essay right at the outset: