 Along with his efforts to secure a place for genuine freedom of the will in his metaphysics, Crusius also emphasizes the limits of the human understanding. Crusius claims that the acts of the understanding are governed by principles, including a formal principle (the principle of contradiction), but also and importantly a number of “material” principles that govern what is thinkable for the human mind (where what is unthinkable is not for that reason contradictory). Among these principles are the principle of inseparability, according to which that which cannot be separated in thought cannot be separated in fact, and the priniciple of incombinability, according to which that which cannot be combined in thought cannot in fact be combined (Crusius 1743: §XXVII; 1745: §15). Significantly, and drawing on Thomasius (via Hoffmann 1737), Crusius contends that these principles which together constitute the “essence of the understanding” supply us with a criterion of truth, which consists in the agreement of thoughts with things, such that only that can be true which conforms to the principles of human understanding (Crusius 1745: §15; 1747: §51). These material principles are subsequently used by Crusius to vindicate a variety of substantive metaphysical claims, including one he dubs the principle of sufficient cause (Crusius 1745: §31). On this basis, Crusius erects his own distinctive metaphysics, proceeding through the now-established Wolffian topics, though Crusius assigns natural theology a place more befitting its importance and ignores empirical psychology because it takes up spontaneous and free acts of the mind and so does not concern necessary truths of reason, the only proper subject of metaphysics.