Crusius was professor of philosophy at Leipzig, and is principally remembered as one of the targets of Kant, particularly in his Dreams of a Spirit-Seer (1766). Crusius himself was a pietist, whose opposition to Wolff lay in admitting moral and other intuitive principles into the foundations of knowledge, without over-concern about their origin and authority. However Crusius also rejected the traditional arguments, especially the ontological argument for the existence of God, and stressed the limits of human understanding, as well as the impossibility of underpinning it by purely logical and mathematical means. In these respects and others he was an important influence on Kant.