Hume wrote that all our ideas are caused by sense impressions, thus originating in the external world. This means that all our knowledge is ultimately derived from experience. Systems of rational metaphysics attach to nothing in the real world; they contain nothing but “sophistry and illusion.” Hume’s empiricism was a rude awakening to Kant, who was trained in the rationalist philosophy of Leibniz.

Kant admits that Hume “interrupted my dogmatic slumber and gave a completely different direction to my researches…” Particularly affronting to Kant was Hume’s argument that the necessary relationship between cause and effect has no basis in experience, and thus cannot be taken as universally valid. Although we repeatedly observe effect B being produced by cause A, we cannot definitively state that A necessarily causes B.

To Kant, this represented an attack on the very idea of a priori knowledge. A priori knowledge is independent of experience. If something is knowable a priori, it is necessarily and universally true. Consider the phrase “all bachelors are unmarried men” – there is no way in which this statement cannot be true. Conversely, a posteriori knowledge is based on experience. It is contingent and “comparatively universal”. For example, we know from experience that it takes 20 minutes to get from home to work in the morning. But this is not necessarily true; it could be otherwise. Its truth is contingent on a whole host of factors: the traffic, how fast we travel, the available routes, and so on. To these categories of knowledge, Kant adds two further distinctions: analytic and synthetic judgements.

Analytic judgements are when predicate B is contained within subject A. Again, the quality (predicate) of being unmarried is contained within the subject “bachelor”. The statement “all bachelors are unmarried men” is analytic, for it yields no new knowledge, it merely elucidates our existing concepts.

Synthetic judgements add two concepts together. In other words, they synthesise objects of our knowledge together to form a new understanding. Synthetic judgements create new knowledge. To give Kant’s own example, we cannot know a priori that bodies are heavy. Although this in fact turns out to be true, we can only know it from experience. Thus, the quality of being heavy is attached to the subject “body”.

It might seem that the analytic/synthetic distinction runs parallel to the a priori/ a posteriori distinction, raising the question of why Kant bothered introducing it. This, however, is not the case. Kant’s purpose in the Critique of Pure Reason is to ask: “How are synthetic judgements a priori possible?” In other words, can we yield new knowledge using pure reason alone? Kant thinks we can, although I am not yet convinced. I will keep you posted as I discover more!