If time and space, among other things, are constructs of the mind, we might wonder what is actually “out there,” independent of our minds. Kant answers that we cannot know for certain. Our senses react to stimuli that come from outside the mind, but we only have knowledge of how they appear to us once they have been processed by our faculties of sensibility and understanding. Kant calls the stimuli “things-in-themselves” and says we can have no certain knowledge about their nature. He distinguishes sharply between the world of noumena, which is the world of things-in-themselves, and the world of phenomena, which is the world as it appears to our minds.
After giving what he considers a satisfactory account of how synthetic a priori knowledge makes mathematics and science possible, Kant turns to metaphysics. Metaphysics relies on the faculty of reason, which does not shape our experience in the way that our faculties of sensibility and understanding do, but rather it helps us reason independent of experience. The mistake metaphysicians typically make is to apply reason to things in themselves and try to understand matters beyond reason’s grasp. Such attempts tend to lead reason into contradiction and confusion. Kant redefines the role of metaphysics as a critique of pure reason. That is, the role of reason is to understand itself, to explore the powers and the limits of reason. We are incapable of knowing anything certain about things-in-themselves, but we can develop a clearer sense of what and how we can know by examining intensively the various faculties and activities of the mind.
In the Critique of Pure Reason, Kant achieves a synthesis between the competing traditions of rationalism and empiricism. From rationalism, he draws the idea that pure reason is capable of significant knowledge but rejects the idea that pure reason can tell us anything about things-in-themselves. From empiricism, he draws the idea that knowledge is essentially knowledge from experience but rejects the idea that we can infer no necessary and universal truths from experience, which is Hume’s conclusion. As a result, he avoids the metaphysical speculations of the rationalists, for which any definite proof seems unattainable but maintains the rationalists’ ambitious agenda, which attempts to give some answer to the sorts of questions that inevitably occur when we think philosophically. By locating the answers to metaphysical questions not in the external world but in a critique of human reason, Kant provides clear boundaries for metaphysical speculation and maintains a sensible, empirical approach to our knowledge of the external world.
Kant achieves what he calls a Copernican revolution in philosophy by turning the focus of philosophy from metaphysical speculation about the nature of reality to a critical examination of the nature of the thinking and perceiving mind. In effect, Kant tells us that reality is a joint creation of external reality and the human mind and that it is only regarding the latter that we can acquire any certain knowledge. Kant challenges the assumption that the mind is a blank slate or a neutral receptor of stimuli from the surrounding world. The mind does not simply receive information, according to Kant; it also gives that information shape. Knowledge, then, is not something that exists in the outside world and is then poured into an open mind like milk into a cup. Rather, knowledge is something created by the mind by filtering sensations through our various mental faculties. Because these faculties determine the shape that all knowledge takes, we can only grasp what knowledge, and hence truth, is in its most general form if we grasp how these faculties inform our experience.
The lynchpin to Kant’s critical philosophy is his category of the synthetic a priori. Although distinctions similar to Kant’s a priori–a posteriori distinction and his synthetic–analytic distinction have been made by thinkers such as Hume and Leibniz, Kant is the first to apply two such distinctions to generate a third category for knowledge. Hume, for instance, does not distinguish between what Kant calls the analytic and the a priori and what he calls the synthetic and the a posteriori, so that, for Hume, all synthetic judgments are necessarily a posteriori. Since only a priori truths have the important qualities of being universal and necessary, all general truths about reality—as opposed to particular observations about unconnected events—must be a priori. If our a priori knowledge is limited to definitional analytic judgments, then Hume is right in concluding that rationally justified knowledge of universal and necessary truths is impossible. Kant’s coup comes in determining that synthetic judgments can also be a priori. He shows that mathematics and scientific principles are neither analytic nor a posteriori, and he provides an explanation for the category of the synthetic a priori by arguing that our mental faculties shape our experience.
Kant differs from his rationalist predecessors by claiming that pure reason can discern the form, but not the content, of reality. Rationalists, such as Descartes, Spinoza, and Leibniz, speculated about the nature of time, space, causation, God, and the universe, and they believed at least on some level that they could come up with relatively confident answers through the exercise of pure reason. Kant, who was educated in this tradition, argues that his predecessors have not given any clear grounding for their metaphysical speculation, but that is because they assume that time, space, causation, and the like are the content of an external reality that the mind must reach out and grasp. Kant turns this assumption on its head, suggesting that time, space, and causation are not found in experience but are instead the form the mind gives to experience. We can grasp the nature of time, space, and causation not because pure reason has some insight into the nature of reality but because pure reason has some insight into the nature of our own mental faculties.