“Every event must have a cause” cannot be proven by experience, but experience is impossible without it because it describes the way the mind must necessarily order its representations. We can understand Kant’s argument again by considering his predecessors. According to the Rationalist and Empiricist traditions, the mind is passive either because it finds itself possessing innate, well-formed ideas ready for analysis, or because it receives ideas of objects into a kind of empty theater, or blank slate. Kant’s crucial insight here is to argue that experience of a world as we have it is only possible if the mind provides a systematic structuring of its representations. This structuring is below the level of, or logically prior to, the mental representations that the Empiricists and Rationalists analyzed. Their epistemological and metaphysical theories could not adequately explain the sort of judgments or experience we have because they only considered the results of the mind’s interaction with the world, not the nature of the mind’s contribution. Kant’s methodological innovation was to employ what he calls a transcendental argument to prove synthetic a priori claims. Typically, a transcendental argument attempts to prove a conclusion about the necessary structure of knowledge on the basis of an incontrovertible mental act. Kant argues in the Refutation of Material Idealism that the fact that “There are objects that exist in space and time outside of me,” (B 274) which cannot be proven by a priori or a posteriori methods, is a necessary condition of the possibility of being aware of one’s own existence. It would not be possible to be aware of myself as existing, he says, without presupposing the existing of something permanent outside of me to distinguish myself from. I am aware of myself as existing. Therefore, there is something permanent outside of me.
This argument is one of many transcendental arguments that Kant gives that focuses on the contribution that the mind itself makes to its experience. These arguments lead Kant to conclude that the Empiricists’ assertion that experience is the source of all our ideas. It must be the mind’s structuring, Kant argues, that makes experience possible. If there are features of experience that the mind brings to objects rather than given to the mind by objects, that would explain why they are indispensable to experience but unsubstantiated in it. And that would explain why we can give a transcendental argument for the necessity of these features. Kant thought that Berkeley and Hume identified at least part of the mind’s a priori contribution to experience with the list of claims that they said were unsubstantiated on empirical grounds: “Every event must have a cause,” “There are mind-independent objects that persist over time,” and “Identical subjects persist over time.” The empiricist project must be incomplete since these claims are necessarily presupposed in our judgments, a point Berkeley and Hume failed to see. So, Kant argues that a philosophical investigation into the nature of the external world must be as much an inquiry into the features and activity of the mind that knows it.
The idea that the mind plays an active role in structuring reality is so familiar to us now that it is difficult for us to see what a pivotal insight this was for Kant. He was well aware of the idea’s power to overturn the philosophical worldviews of his contemporaries and predecessors, however. He even somewhat immodestly likens his situation to that of Copernicus in revolutionizing our worldview. In the Lockean view, mental content is given to the mind by the objects in the world. Their properties migrate into the mind, revealing the true nature of objects. Kant says, “Thus far it has been assumed that all our cognition must conform to objects” (B xvi). But that approach cannot explain why some claims like, “every event must have a cause,” are a priori true. Similarly, Copernicus recognized that the movement of the stars cannot be explained by making them revolve around the observer; it is the observer that must be revolving. Analogously, Kant argued that we must reformulate the way we think about our relationship to objects. It is the mind itself which gives objects at least some of their characteristics because they must conform to its structure and conceptual capacities. Thus, the mind’s active role in helping to create a world that is experiencable must put it at the center of our philosophical investigations. The appropriate starting place for any philosophical inquiry into knowledge, Kant decides, is with the mind that can have that knowledge.
Kant’s critical turn toward the mind of the knower is ambitious and challenging. Kant has rejected the dogmatic metaphysics of the Rationalists that promises supersensible knowledge. And he has argued that Empiricism faces serious limitations. His transcendental method will allow him to analyze the metaphysical requirements of the empirical method without venturing into speculative and ungrounded metaphysics. In this context, determining the “transcendental” components of knowledge means determining, “all knowledge which is occupied not so much with objects as with the mode of our knowledge of objects in so far as this mode of knowledge is to be possible a priori.” (A 12/B 25)
The project of the Critique of Pure Reason is also challenging because in the analysis of the mind’s transcendental contributions to experience we must employ the mind, the only tool we have, to investigate the mind. We must use the faculties of knowledge to determine the limits of knowledge, so Kant’s Critique of Pure Reason is both a critique that takes pure reason as its subject matter, and a critique that is conducted by pure reason.
Kant’s argument that the mind makes an a priori contribution to experiences should not be mistaken for an argument like the Rationalists’ that the mind possesses innate ideas like, “God is a perfect being.” Kant rejects the claim that there are complete propositions like this one etched on the fabric of the mind. He argues that the mind provides a formal structuring that allows for the conjoining of concepts into judgments, but that structuring itself has no content. The mind is devoid of content until interaction with the world actuates these formal constraints. The mind possesses a priori templates for judgments, not a priori judgments.