Contrary to Hume, Kant argued that synthetic a priori judgments are possible in mathematics and physics. But are they possible in metaphysics? Kant was pessimistic about the ability of human reason to acquire theoretical knowledge of any reality lying beyond the boundaries of human experience. According to Kant, one cannot know things-in-themselves.

Kant maintained that one could have knowledge of causality in the realm of appearances. In fact, the rationalist and empiricist traditions begun by both René Descartes and John Locke had both assumed there was a dichotomy between ideas about reality and the real world itself. How could one be sure that the ideas in one's mind correspond to the real world? One cannot. Locke adopts the “representative realist” position, since he thinks ideas represent primary qualities.