 But if mathematics does not provide a model for a genuinely scientific metaphysics, the relation between metaphysics and the empirical sciences is also unpromising. In the first place, Kant has argued that experience cannot reveal metaphysical entities. We could never know, for instance, that we are free: like everything else we can know, human conduct is in principle open to fully determinate causal explanation. Second, experience cannot generate the sort of necessity Kant associates with metaphysical conclusions. (This is a long-standing bone of contention between Humean and Kantian accounts of knowledge—for instance, as regards causation. See the entry on  Kant and Hume on causality.)  That is, our investigation of the world, no matter how systematic or scientific, only reveals contingent facts: it cannot show that such-and-such must be the case. To hold that scientific laws have the quality of necessity—so that they really are laws, and not mere generalizations or rules of thumb—is a metaphysical rather than an empirical claim.