Knowledge is “synthetic” or “ampliative,” according to Kant, if it is about objects that can be experienced in the world. It is a priori if it can be known without experience. Kant’s motivating metaphysical question was, “How is it possible to know certain principles about the world, without prior experience?”

Kant’s solution was to apply a “transcendental deduction” to such principles and show that without them experience would not be possible. For example, concerning causation, he argued that consciousness itself requires orderly experience based on necessary connections in reality. This was Kant’s answer to David Hume’s (1711–1776) reduction of causation to constant conjunction. He rejected Hume’s skepticism that constant conjunction is all that there is by claiming that the world could only make sense to us if we assumed that that there were real causal connections in it. In his Prolegomena to Every Future Metaphysics (1783), Kant famously said that Hume had awakened him from his “dogmatic slumbers.”