The Critique of Pure Reason is one of the most important philosophical texts ever written. Like Copernicus, Kant dared to question the ordinary perspective from which we habitually view the world. While we usually imagine that knowledge occurs when the mind corresponds to objects, Kant argues that knowledge is made possible by the fact that objects must conform to the shape of our minds. This results in a moderate form of skepticism, known as "transcendental idealism," whose primary tenet is that we cannot know things as they are in themselves because we only know things as they appear to us. Kant's thesis had a monumental influence on the culture of the last two centuries, giving rise to cultural movements and theoretical approaches including: German Idealism, Romanticism, Modernism, Marxism, Existentialism, Psychoanalysis, Structuralism, Post-Structuralism, and even Quantum Physics.

Immanuel Kant (1724-1804) has been caricatured as a stiff German professor, whose Stoic habits were so predictable that the people of Königsberg, his hometown, could set their clocks by his daily walks. Kant's life is best described as a heroic struggle to discover order within chaos or, better, an effort to fix human thought and behavior within it proper limits. He lived and worked during the revolutionary period known as the Enlightenment, a time when political, religious, and intellectual freedom erupted across the Western world. The Critique of Pure Reason was published in the 1780s, between the American and French Revolutions. The first edition (1781) came out during the reign in Prussia of the "enlightened despot," Frederick the Great, to whom Kant dedicated hisfamous essay "What is Enlightenment?" in 1784. The second edition (1787) was issued during the reign of Frederick's reactionary heir, Frederick William II. During the 1790s Prussian censors rebuked Kant for the unorthodox implications of his philosophy of religion, which held that God was merely a moral postulate whose existence could not be proven by science. Although Kant's ideas were radical, he did not want to inspire anarchy or actual revolution. For Kant, the freedom of Enlightenment was to be directed toward wisdom, law, and order, not toward license and disorder.

Kant is supposed to have written the first edition of the Critique in 1780 in a matter of months, "an accomplishment," according to Ernst Cassirer, "that is scarcely rivaled, even as a purely literary feat, in the entire history of thought." After the first Critique, Kant published two other Critiques: the Critique of Practical Reason (1788) and the Critique of Judgment (1790). The scope of these texts is remarkable. They deal with metaphysics, epistemology, ethics, and aesthetics. Together the three works form what is known as the "critical" philosophy, the basic approach of which is methodological criticism. In general, in the critical works, Kant asks the question of what we are entitled to claim that we know. While Kant produced his three major Critiques in his 50s and 60s, he continued to write and publish well into his 70s, producing works on a wide range of topics including politics, religion, anthropology, and history.

As part of his struggle to enlighten his age, Kant issued an easier distillation of the content of the Critique of Pure Reason under the title Prolegomena to Any Future Metaphysics (1783). The titles of both of these works are instructive. They indicate that Kant's project is preparatory. He is clearing the ground and criticizing reason in an effort to prepare the way for the future completion of philosophy. A critique of reason is not a system of philosophy, as such a system had been traditionally conceived. He makes this explicit in the often-overlooked second major section of the Critique, "The Transcendental Doctrine of Method." Here he indicates that the problem of the book is that reason needs "discipline," and he argues that until such discipline has been instituted, philosophy, as a finished system of knowledge will not exist. Thus the critical approach is designed to help us learn to philosophize. "In other words, we can only exercise our powers of reasoning accordance with general principles, retaining at the same time, the right of investigating the sources of these principles, of testing, and even of rejecting them." The Critique enacts this process of investigating and testing principles, while encouraging each of us to judge for ourselves using our own critical powers.

The enlightenment facilitated by the critical philosophy is connected to political enlightenment and the liberal ideal of freedom of thought. The critical method assumes that reason is an authority found in each and every human being and that each has a right to openly express objections and doubt. "This privilege forms part of the native rights of human reason, which recognizes no other judge than the universal reason of humanity; and as this reason is the source of all progress and improvement, such a privilege is to be held sacred and inviolable." Political enlightenment facilitates the completion of the philosophical task by setting us free to complete the ground-clearing project of the Critique. Likewise, reason that is improved through critique is supposed to lead to progress in all aspects of human life.

The motto of Kant's 1784 essay, "What is Enlightenment?" was sapere aude ("dare to be wise"). This motto can also serve to guide readers of the Critique. For Kant, enlightenment means that every man has the right to understand the necessary limits of human knowledge. But this right comes with the responsibility to respect those limits and admit the truth. Indeed, in his moral and political writings Kant emphasized obedience to the moral law and to political authority. The idea of obedience to the law is also seen in the Critique of Pure Reason, where Kant catalogues the principles that govern the mind and encourages us to obey them. The problem to be remedied by the critical process is the tendency of those who are interested in metaphysical questions to become undisciplined as they allow themselves to transgress the limits of what the human mind can actually know. If we would dare to be wise, we must admit that there are some things the mind cannot know.

Kant thus shares the epistemological and methodological concerns of modern philosophers such as Descartes and Hume. Indeed, it is not too much to say that Kant redefines metaphysical problems as problems of epistemology. What this means is that Kant transforms questions about the existence of God, about the freedom of the will, and about the nature of time into questions about knowledge. For Kant the crucial question is what any human being could know about God, freedom, and time. Kant concludes that we cannot experience an object such as God as He would be "in Himself" because everything we know comes to us through the filters of experience. Thus God is a concept or idea that we construct out of the tendency of reason to look for absolute and unconditioned things. But, we cannot know of God as He would be outside of our experience. We cannot know, then, that God exists; but neither can we know that God does not exist. Both the dogmatic theist and the dogmatic atheist go beyond the bounds of what is knowable. At best we know that God is an idea-the idea of absolute, unconditioned Being-toward which reason pushes us. Kant more explicitly avoids atheism by claiming that there are certain "transcendental" or "regulative" ideas that we must postulate in light of our practical interests. These ideas-or moral postulates-include the ideas of God, freedom, and immortality. To be honest we must admit that we have no knowledge of these things, even while recognizing that our practical moral interests demand that we believe them. In regard to these issues, Kant clearly states his position in the Preface to the Second Edition of the Critique: "I have found it necessary to deny knowledge, in order to make room for faith."

Kant's project can be understood in relation to the concerns of both Plato and Christian philosophers such as Augustine. Plato and Augustine both assume that there is a difference between what is real and what is merely apparent. With Plato, philosophy-rigorous methodical thinking-is supposed to lead beyond the shadow world of appearances and unfounded opinions toward the world of reality and true knowledge. Augustine and the Christians add that faith is also necessary for this ascent from appearance to reality. Kant's important conclusion is that there is no escape from the world of the mind. The things we experience are shaped by the forms and categories by which the human mind necessarily experiences anything. We cannot, then, experience "things in themselves"-whether these are Platonic Ideas or the Christian God-as they would be if the human mind were not actively working to make sense of them.

While this sounds skeptical, Kant is only agnostic about our knowledge of metaphysical objects such as God. And, as noted above, Kant's agnosticism leads to the conclusion that we can neither affirm nor deny claims made by traditional metaphysics. But Kant is not skeptical about empirical objects and the laws that govern their appearance. Indeed, Kant's effort was explicitly directed to overcoming skepticism about empirical reality of the sort propounded by Hume. Kant famously claims in the Introduction to the Prolegomena that Hume was the philosopher who had "awakened him from his dogmatic slumbers." Hume had argued that the empirical world of experience was not governed by necessity because the laws of nature were mere generalizations constructed by us as we tried to make sense of experience. But this empirical point of view created profound skepticism about the nature of scientific law. Could it be possible, Kant wondered, that the laws of nature and even the laws of mathematics were not governed by necessity? One of Kant's goals is to show us that science is a valuable way of understanding the world of experience, precisely because science constrains itself to the world of experience and does not stray into metaphysical speculation. The necessary laws of experience are truly necessary because any experience must conform to them. In other words, it would be impossible to imagine an experience that was not constrained by the laws of experience.

Furthermore, Kant claims that the scientific method-as utilized by Galileo and Copernicus-provides a model for the way in which we should understand the process of reason. Scientists propose experiments and understand the world in terms of the answers that are received in light of specific questions and hypotheses. This shows us that the mind is active in comprehension and not passive. We approach the world with questions and actively look for answers. In the same way, Kant argues that we impose form, categories, and structures upon the sense data that we receive. Ultimately, what science discovers are the necessary ways in which the world of experience is structured.

An example might help. The first section of the book, the "Transcendental Aesthetic," argues that space and time are the necessary conditions for the possibility of any experience. Space, Kant says, is the form of outer sense, while time is the form of inner sense. We know nothing about space and time in themselves, apart from our experience. We do know, however, that they are presupposed in any possible experience. This is what Kant means when he say that time and space are "empirically real" but "transcendentally ideal": time and space are necessary components of any experience-they are empirically real, even though we know nothing about them in themselves-and they are transcendentally ideal.

An argument such as this, about the necessary conditions of experience, is what Kant calls "transcendental": it is an a priori argument about that which constitutes experience. "I apply the term transcendental to all knowledge which is not so much occupied with objects as with the mode of our cognition of these objects, so far as this mode of cognition is possible a priori." By, a priori, Kant means anything that is not established by way of inductive generalizations based upon experience. Inductive generalizations-such as, "heavy objects tend to fall"-are, in Kant's language, a posteriori. Unlike inductive generalizations, a priori reasoning focuses on the very constitution of experience: it postulates necessary and universal features of any possible experience that cannot themselves be confirmed by experience. For example, the claim that "every change must have a cause" is a necessary assumption of any experience. In other words, it is impossible to imagine a change in experience that is not caused, just as it is impossible to imagine an experience that did not occur in space or time.

Of course, one can raise objections to Kant's theory. The most important objection focuses on a subjectivist interpretation of Kant's "transcendental idealism." For Kant, subjective conditions constitute objectivity. Although he postulates the existence of a "thing-in-itself, " Kant tells us that the thing-in-itself is a "thing = X" about which we can know nothing. This leaves us, unfortunately, divorced from the world in disquieting ways. Later philosophers respond to Kant's subjectivism either by arguing for a more complete form of idealism as Fichte and Hegel were to do, by claiming that the essence of the thing-in-itself is Will, as Schopenhauer was to do, or by explicitly affirming subjectivism as Nietzsche was to do. In the twentieth century, the subjective focus that is present in Kant's philosophy would give way to pragmatism, phenomenology, existentialism, and to Kuhnian philosophy of science.

Kant's virtue, however, is that he was not content merely to propound a solipsistic form of subjective idealism. Rather, he recognized the importance of intersubjectivity and maintained that the empirical world will be experienced by everyone according to the same laws. Indeed, he acknowledged that experience is not something we produce as if we were gods. The sensible manifold is given to us, we know not how. But this world of sense is comprehensible because it conforms to the necessary conditions of experience. Kant admits that the human mind is creative, as the Romantics and Modernists who followed Kant would emphasize. But Kant emphasizes that the mind must respect the laws which govern its own creativity. Kant's text is seminal because of his unique approach to the discipline of philosophy. It remains vital because it celebrates the power of the human mind.

Andrew Fiala is Associate Professor of Philosophy and Humanistic Studies at the University of Wisconsin-Green Bay. He is the author of The Philosopher's Voice and articles on topics in nineteenth-century philosophy, ethics, and political philosophy.