For thinkers like Aristotle and Thomas Aquinas, what we know first and most fundamentally is the world outside the mind—sun and clouds, rocks and trees, tables and chairs, dogs and cats and people. Reflection on this knowledge leads us to inquire into the mental acts and powers by which we acquire such knowledge, and then in turn into the nature of the mind itself. But the objective, natural order is primary. The subjective world of human thought and experience can properly be understood only by fitting it into this larger context.

Ancient and medieval philosophy in general tended to share this emphasis on the priority of the objective world. One of the distinguishing features of modern philosophy is its reversal of this order of things. For the moderns, it is the subjective realm of the mind that we know first and best. Only after we determine its nature and powers can we go on to investigate the world outside the mind—if indeed there is any such world. René Descartes, the father of modern philosophy, set the theme in the 17th century with his famous Cogito, ergo sum (“I think, therefore I am”). That I exist as a thinking thing could, he argued, be known with certainty even if my senses were entirely untrustworthy and the world outside me were a mere dream. The self thus became the Archimedean point from which pure reason must proceed to deduce the nature and reality of anything else.

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Because Descartes took the self’s knowledge to begin with what can be established by reason alone apart from sensory experience, his approach to epistemology (the branch of philosophy concerned with the nature of knowledge) came to be known as rationalism. By contrast, the rival empiricist tradition represented by John Locke held that knowledge must begin with sensory experience. But for modern empiricists, all the self can directly know by way of sensory experiences are just the experiences themselves, rather than the world of external material objects that common sense takes us to see, hear, taste, touch, and smell. Whether there really is such a world beyond the “veil of perceptions” has to be established by argument. Like the rationalists, then, the empiricists took the subjective world to be primary.

Having locked itself in this prison of subjectivity, early modern philosophy soon found that it had lost the keys and could not get back out. The rationalist tradition culminated in the early 18th century in Gottfried Wilhelm Leibniz’s thesis that the physical world is nothing more than a kind of virtual reality collectively experienced by all minds. This is a variation on idealism, the view that minds and their ideas are all that exist. To be sure, this included more than just human minds. Rationalists developed highly ambitious systems of metaphysics (the subdiscipline of philosophy devoted to the investigation of the general structure of reality and ultimate causes of things). This encompassed proofs of the existence of God, conceived of in the case of Descartes and Leibniz as the supreme mind that creates all other minds. But in Leibniz’s denouement of rationalism, matter was left with no role to play.

The empiricist tradition also led in the 18th century, at first, to idealism coupled with theism, in the philosophy of George Berkeley. If there is no knowledge apart from perception and all we perceive are our own experiences themselves, then, Berkeley concluded, we can never know (indeed, cannot so much as conceive of) a material world that lies beyond those experiences. The reason my stream of experiences keeps in sync with yours and everyone else’s is not that there is a common material world we all inhabit, but rather because there is a supreme mind, God’s, that coordinates the experiences of all the others.

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But though this deletes matter from our picture of the world, it leaves intact enduring substances of an immaterial kind—your mind, mine, and everyone else’s, together with God’s—and the causal relations that hold between our choices and their outcomes, and between God and the minds he creates and coordinates. David Hume showed that a consistent empiricism will have to do away with all of this as well. Again, all we can know, on this view, are our own experiences—for example, my experience of seeing a cue ball knock into an eight ball, followed by my experience of the eight ball falling into the corner pocket. But what I cannot experience, according to Hume, is any causal connection between these events. One event simply follows the other, and that is all that I can say. I have no more basis for believing that the one caused the other than I have for believing that the fact that I woke up before sunrise shows that my waking up caused the sun to rise. Moreover, I have, on Hume’s view, no empirical basis for believing that there is some enduring substance, a self, that has all of these experiences. For again, what I am aware of in perception is just the succession of experiences themselves, and not some underlying entity in which the experiences inhere.

These results undermine ambitious metaphysical arguments of the kind by which Leibniz and Berkeley attempted to prove God’s existence. If we cannot know causal relations, we cannot reason to God as first cause. But they also undermine science, which is supposed to be in the business of establishing law-like causal relations between events. Indeed, they leave us with a thoroughgoing skepticism which denies us knowledge even of the self, let alone any world beyond the self. Thus did the dialectic between rationalism and empiricism not only collapse the objective physical world into the subjective realm of the mind, but also threatened to dissolve the latter as well.

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Enter Immanuel Kant, whose philosophy is expounded with uncommon and admirable lucidity in the new book Kant: A Revolution in Thinking by Marcus Willaschek, a philosophy professor at Goethe University Frankfurt. Like his early modern predecessors, Kant begins with the primacy of the subjective point of view of human thought and consciousness, rather than objective physical reality. And he hoped to preserve what he took to be of value in both the rationalist and empiricist ways of developing this idea. But he wanted to do so in a way that avoided idealism’s denial of mind-independent reality and Hume’s skepticism. The resulting position was spelled out with such systematic brilliance that it would widely come to be regarded as the paradigmatic expression of the modern approach to epistemology and metaphysics.

Whether it actually works is another matter. The basic idea is as follows. With the rationalists, Kant holds that no knowledge would be possible unless certain notions were built into the very structure of the human mind prior to sensory experience. These include time and space, along with what Kant calls the “categories”—unity, plurality, substance and accident, cause and effect, and eight others. Apart from these, what enters the mind by way of sensory experience would be an unintelligible jumble, like the static on an old television set that has lost its signal. The mind imposes order on this input by organizing it into multiple distinct objects occupying three dimensions and persisting from moment to moment, changing and being changed by other objects, and so on.

Contrary to the rationalists, however, Kant denies that these innate concepts could by themselves ever give us knowledge, any more than a set of cookie cutters could give us cookies in the absence of dough. He agrees with the empiricists that sensory input is a necessary condition for knowledge, even if it too is not sufficient. As Willaschek summarizes the point:

Cognition and knowledge, Kant states, require both sensory perception (intuition) and abstract concepts: “Thoughts without content are empty, intuitions without concepts are blind,” runs a famous and oft-quoted dictum of his. The first part of this proposition is directed at rationalism, and the second at empiricism.

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With this, Kant claimed, we have a bulwark against the skepticism that Hume showed to be empiricism’s sequel. If all we had was raw sensory input (“intuitions,” in Kant’s jargon) then we would indeed never be able to get beyond it to knowledge of a world of mind-independent and causally related physical objects. The rational foundations of common sense and science alike would disintegrate. But because this otherwise “blind” input is filtered through innate concepts like time, space, substance, causality, and the like, we are presented instead with the objects and events of everyday experience and can go on to study them scientifically.

At the same time, Kant, like Hume, nevertheless wants to clip the wings of ambitious metaphysical speculation. Because concepts like substance and causality are hardwired into the mind prior to experience, they are immune to empiricist attack. But they nevertheless can be applied only together with experience, and thus, Kant concludes, they can never yield knowledge of anything beyond the world of experience. For example, we cannot deploy the idea of causality to argue for the existence of a divine cause lying outside the world of experience. Nor can we use the idea of substance to argue for the existence of an imperceptible immaterial substance or soul. Kant’s philosophy aims to give us back the objective physical world that the rationalists and empiricists took from us. But it takes away from us the knowledge of God and the soul that some of them offered in consolation.

Yet even this already questionable exchange turns out to be something of a bait and switch. For what we can know about the objective physical realm on Kant’s account is much less than meets the eye. Time, space, substance, causality, and so on have, as it turns out, nothing to do with the way the world outside us really is in itself. Because they are simply the filters through which the mind takes in sensory input, they reflect merely the way things appear to us. They are no more part of objective reality than the red you see when looking through rose-colored glasses is really there in the objects you’re looking at. In fact, according to Kant, we can know nothing at all about what the world outside the mind really is like in itself. We can say only that it is there. Other than that, we know only how it appears to us.

And things may be even worse than that. For on one natural reading of Kant, the way the objective world as it is in itself relates to the subjective appearances through which it is represented to us, is by causing those appearances. Yet causality is supposed to be a category that, in Kant’s view, does not apply to the external world as it is in itself. In that case, it is not correct to think of that world as causing our subjective experiences of it. But then, how can we know it really exists? Kant’s position seems, after all is said and done, to lead us back to skepticism, or at best to another variation on the idealist position that the physical world exists only relative to the mind’s awareness of it. As Willaschek notes, there are alternative ways of interpreting Kant. But it is very difficult to make any of them work, and it is no surprise that Western philosophy in the century after Kant went precisely in an idealist direction.

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This is well-trodden ground to those familiar with Kant, but for those who are not, Willaschek provides an exposition that is as accessible as possible without oversimplifying. His book is also unusual in not beginning his account of Kant’s philosophy with his epistemology and metaphysics (as I have done here myself), but instead with Kant’s political and moral philosophy. This is not, in my view, the most natural approach, insofar as Kant’s approach to ethics is, I think, best understood in light of his metaphysics and epistemology rather than the other way around. But it is a defensible and interesting approach, and it is certainly sure to make Kant less forbidding to the uninitiated reader. It also allows Willaschek more easily to develop one of the book’s central themes, which is that Kant’s philosophical motivations were primarily practical rather than theoretical—a plausible enough thesis given the severe limits on human knowledge entailed by his philosophy.

Like his metaphysics, Kant’s ethics begins, not with reality outside the mind, but rather with how the mind must think about reality. The difference is that ethics is not a matter of theoretical reason (which is concerned with how things are) but rather of practical reason (which is concerned with what one ought to do). In a line of argument that is no less clever or influential than his theory of knowledge, Kant essentially attempts to spin out the core of an entire system of morality from the idea of morality itself.

The basic strategy works like this. An act has moral value, in Kant’s view, only if it is done out of the motive of duty. If it is done for some other reason (such as self-interest or even benevolence) it is not necessarily wrong, but neither is it praiseworthy. It is only if I do the right thing simply because I ought to do it that I can be worthy of moral praise. But acting out of duty is essentially a matter of acting out of respect for the moral law. And a law is of its nature something that applies universally—to all people, and under all circumstances. This, all by itself, gives one a way of determining whether an action is right. I simply ask whether, in doing it, I would be obeying a true law, a principle that could apply to all people and under all circumstances. The idea is enshrined in Kant’s famous “Categorical Imperative,” the first formulation of which says: act only according to a principle that you could will to become a universal law.

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Kant famously illustrates the idea by considering a case where one wonders whether one ought to break a promise he has made, because it would inconvenience him to keep it. To do this, Kant argues, would be to follow a principle that could *not *be made a universal law. For if all people followed a principle that would allow breaking a promise whenever it is convenient to do so, the whole institution of making promises would disappear, since no one would be able to trust anyone who made a promise. There is an incoherence in such a principle, insofar as it both presupposes the institution of making promises while at the same time undermining it. So, since one could not be following a true law when breaking a promise, one should not break one’s promises. Kant thinks the core of traditional morality (such as rules against murder, stealing, adultery, and the like) can all be derived by such applications of his principle.

Strikingly, Kant holds that by determining the content of morality in this way, human reason is essentially legislating the moral law for itself. This may seem to make morality subjective, just as Kant’s epistemology seems to make physical reality subjective. In both instances, though, Kant insists that the opposite is the case. Where physical reality is concerned, he argues that imposing the concepts of space, time, and the categories on it is not merely something that this or that human mind happens to do, but rather something that all human minds as such must of necessity do. And where ethics is concerned, the moral law is not what this or that rational being happens to legislate, but rather what all rational beings must of necessity legislate. In Kant’s view, if we’re correctly applying the test of the Categorical Imperative, we’ll all get the same answers.

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All the same, Kant’s philosophy has a distinctly claustrophobic feel, trapping us as it does in our own minds. Nor does it help to say that all minds must conceive and will the same thing, because this doesn’t guarantee access to an objective world, only a kind of intersubjectivity or collective virtual reality. In the two centuries after Kant, modern philosophy would strip us even of that, as many of his successors argued that the way we conceive and engage morally with the world reflects, not necessary and universal categories built into all human minds as such, but merely contingent and local assumptions determined by ever-changing biological, historical, cultural, or economic circumstances. Kant himself was by no means a relativist, but his revolution in philosophy greased the skids for contemporary relativism.

It has also encouraged a kind of idolatry of human beings. Isaac Newton had famously held that space is God’s sensorium, the organ through which he knows the world. As Marcus Willaschek writes, “[I]n Kant, Newton’s sensorium of God has become a sensorium of man.” It is the human mind rather than the divine mind that makes reality. And again, for Kant it also makes the moral law. Mix these themes together with the relativism that has become the de facto philosophy of contemporary Western culture, and we have a recipe for living ungodly lives, but with a godlike arrogance—an outcome that would have horrified Kant, but which he inadvertently helped usher in.

The main weakness of Willaschek’s book is that he fails to consider this particular downside of Kant’s legacy. Though he certainly doesn’t pretend that Kant was perfect, he presents the main lines of his philosophy as salutary, the seeds of what has become the common sense of liberal modernity. No doubt that is because Willaschek regards liberal modernity as an unalloyed good. But even those of us who do not share that judgment can profit from his superb introduction to the thought of one of its chief architects.