Immanuel Kant's Critique of Pure Reason is one of the most influential works in the history of philosophy - not to mention one of the most challenging. Its topic is the nature of human knowledge, and the question of whether or not it is possible to have knowledge of the world at all.

David Hume's 1748 Enquiry Concerning Human Understanding is a modern philosophical classic that helped reshape epistemology - the philosophy of knowledge. It is also a classic of the critical thinking skills of analysis and reasoning. Analysis is all about understanding how arguments work and fit together.