Critique means a detailed analysis and assessment of something, especially a literary, philosophical, or political theory. Before they build a metaphysical system, philosophers must assess the origin and nature of our minds’ mental materials. Imagine you’re a builder living in medieval times. One day, the king summons you to a construction site. He points at a massive pile of building materials and says, “I want you to turn this into a tower that reaches heaven – or as close as possible, at least.” What do you do? Well, you should begin by inquiring into the materials. What are they made of? How strong are they? By answering these questions, you’ll be able to tell how tall you can build the tower. The alternative is to just start building and hope for the best – a recipe for disaster if you end up going higher than the materials can bear.  The same argument applies to philosophers when they want to build a metaphysical system. The key message here is: Before they build a metaphysical system, philosophers must assess the origin and nature of our minds’ mental materials. Metaphysics is the branch of philosophy that tries to elevate our knowledge of the world into the loftiest realms of human inquiry. Using the abstract concepts and logical principles of reason, it attempts to go beyond the empirical evidence of the natural sciences and grasp the ultimate nature of reality. Consider time, for instance. Does it have a beginning? Or does it stretch back into eternity? These are examples of metaphysical questions. Answer a bunch of them, tie your answers together into a coherent body of thought, and you have a metaphysical system. Since the days of ancient Greece, many philosophers have attempted to build various metaphysical systems. But prior to the Critique of Pure Reason, most of them tried to do so without first inquiring into the origins and nature of our minds’ mental materials. They just took the concepts and logical principles they had at hand and started building with them. But are those materials actually fit for the task of metaphysics? And if so, how high do they allow us to build our metaphysical towers, so to speak? If the answer is “all the way to heaven,” then great – build away. But if it’s “not very far,” then we should stay closer to the earth – contenting ourselves with the more mundane knowledge of the sciences, while leaving the more esoteric stuff to religion. Either way, we need to know the answer in advance. Otherwise, we’ll be in danger of building a tower that will crumble beneath our feet.