Summary
Kant now turns to the question of metaphysics. Math is possible because of the pure intuitions of the senses—space and time. The natural sciences are possible because of the pure categories of the understanding. How is metaphysics possible?
The difficulty posed by metaphysics is that, while the first two sciences ultimately rely on the empirical world, either for their concepts or for verification, metaphysics uses concepts that can’t be given in experience—for example God, freedom, the immortal soul. While we can see an object in space, observe it changed by time, or visualize a mathematical problem, we can't point to "God," "freedom," or "the soul." They can’t be confirmed or disproven by empirical experience.
Metaphysics can be understood as Reason’s knowledge of itself, and of objects that are conceived by it. Unlike the understanding, which regulates our experience, Reason is that part of our mind that has no experiential use. On the contrary, it seeks to have knowledge of those things that are beyond experience, to the know the ultimate meaning, or to understand the fundamental possibility of our experience. For that reason, the “ideas” produced by Reason are transcendent. They are ways of trying to grasp something beyond experience that will tell us something about experience itself. But without experience to regulate it, Reason has a tendency to overreach, and is frequently misled.
So the task of the third section will be to explain exactly what the Ideas of Reason are, how they are different from the concepts we derive from experience, and how they interact with the rest of our faculties. This, Kant argues, will help us put these metaphysical "Ideas of Reason" in their proper place—neither letting them run wild (as rationalist philosophers like Leibniz did) or dismissing them altogether (like Hume did).
The first step towards an answer that Kant offers is that Reason is a self-critical mechanism. We can use our Reason to critique itself. Reason can consider its own Ideas, and decide whether or not they can actually be known, instead of chasing after them blindly, as philosophy has done for centuries.
The second, more open-ended answer, is that Reason seeks to “complete” the understanding. Thinking, in an everyday sense, consists of referring the sensory impressions of the intuition to the concepts of the understanding, synthesizing our perception of the world into a stable unity. So long as we keep sensing things, this is a process that will never end. Reason is constantly after objects of knowledge that will give this process a final meaning or purpose. It asks the big questions because it wants to know the world completely, once and for all.
Kant now proposes to critique several Ideas of Reason to demonstrate Reason’s proper use. The first will be the psychological Idea—namely, that in observing our own consciousness, we can have knowledge of some essence of ourselves, our immortal soul. These will be followed by the cosmological Idea—about the nature or purpose of the world, and the theological Idea—the existence of God.
First, the psychological Idea. For centuries, philosophy has wondered whether there is an original being, or existing thing, from which all others are derived. If every object is an effect, there must be an original cause. Is it possible, as Descartes argued, that our own consciousness could be that cause?
Kant says no. Our consciousness is not manifest to us as itself—we’re only aware of it because we can perceive things through it. We can't directly observe it; it manifests only when we notice how other things affect it. When I get hit by a rock, I notice that I am a being who has a body, experiences pain, and (perhaps) feels wronged and wants justice from the person who threw the rock. Our consciousness is only thinkable as a medium for experience; we can’t know our consciousness as something prior to the things that it experiences. And since experience is necessarily finite, we can’t know the immortality of the soul either. In any case, the life of the soul after death could only be known by experience—a contradiction in terms.
Now the cosmological Ideas. To demonstrate the way that Reason oversteps its bounds, and the way that it can check itself against its own irresponsible use, Kant wants to show that, for each Idea, two equally valid arguments can be made for and against each side, thereby demonstrating that something is wrong with the very question itself.
1. The question of whether space and time are infinite or finite fundamentally misunderstands the nature of both, according to Kant. Space and time do not exist “objectively,” but are forms of experience of human consciousness.
2. The same is true of the question of whether the world consists of elements that are simple, i.e., that cannot be divided into composite parts, or whether everything in the world is necessarily composite. Here, the problem is the mistaken idea that “things,” whether as parts or as unified wholes, exist "in themselves." We can have no knowledge of things beyond our experience. Therefore, the objects of experience are neither simple nor composite.
3. The third question, of whether anything in the world is freely caused, or if every event is determined by some pre-existing cause, receives a longer exposition. Here, as with the previous two, the fundamental problems consists of the failure to distinguish between the world of appearances and things in themselves.
As Kant explained in the previous part, causality is a pure concept of the understanding. It is therefore only applicable to the world of sensory experience. In the world of sensory experience, every event has a preceding cause. We must note, also, that causation only applies to the world of sensory experience because it depends on the pure intuition of time—in order for X to cause Y, X must come before Y.
But Reason clues us in to another form of causation, one that that has its origins beyond the sensory world: that of moral obligation, or the ought. By acting in accordance with our sense that we ought to do something, it is no longer an object or phenomenon in our sensory experience that makes something happen, but rather Reason itself. We determine ourselves to act, and so we are removed from the chain of causation of the sensible world. Here we have at last an example of an Idea of Reason determining the understanding.
4. Finally, Kant debunks several arguments for the existence of God. The substance of Kant’s arguments is that, because God has been defined as a being who is beyond all experience, we cannot know him in any substantive way, only an intellectual way. We can entertain this or that argument for his existence, but to know him with the same expectation of universal agreement with which we might say that air is elastic is impossible.
To close, Kant distinguishes metaphysics by pointing out that while mathematics and the sciences are limited, metaphysics is bounded. There are realms of experience that mathematics and the sciences cannot answer, but within them, every question is answerable. For metaphysics, on the other hand, there are questions that it cannot solve. That means that metaphysics is, in a sense, the science of what we cannot know.
Analysis
The third section of the Prolegomena turns its attention to the third faculty of the mind, Reason. (Reason and Idea are capitalized throughout to distinguish these from their ordinary conversational usage.) Kant believes one of the main contributions of the Critique of Pure Reason was to draw a distinction between Reason and the understanding—in effect, between two kinds of abstract thinking—that philosophers before him have lumped together.
The understanding regulates our sensory experience by furnishing us with concepts that we use to connect sensory impressions: this coffee is hot because I saw that it was just brewed. Reason poses questions about what lies beyond, or underneath experience. It tries to give purpose and meaning to what we are experiencing. Reason wants to know whether God exists, whether we have immortal souls, whether we are free acting subjects—questions that, by their definition, cannot be answered by experience. These questions are Ideas of Reason, and the task of this section, as well as the task of metaphysics, will be to determine if these questions can be answered, and what role these ideas play in our daily experience.
Kant’s first answer is that the understanding wants to be completed. On a day to day basis, thinking consists of matching concepts of reason to the sensory impressions. But our mind wants to go beyond this. We want to know what it’s all for, or what the ultimate nature of experience is—a desire that Kant would explore in greater detail in a later work, the Critique of The Power of Judgment. The French philosopher Gilles Deleuze observes that for Kant, Reason is above all a faculty that determines purposes, ends, and values. The desire to know whether there is a God or whether we are free is a way of trying to answer the question of the meaning of our own experience.
The density of Kant’s language and the technicality of his argument makes it easy to overlook what a radical statement this is, particularly for someone living in autocratic Prussia in the 1780s. It is Reason—that is, us ourselves, and not any religious or political authority—that has control over the ultimate purpose of experience. Reason poses these questions, and Reason must clarify them: that means our own intellect is the only truly legitimate authority.
Kant rejects several Ideas of Reason, but there is one that he embraces, and that is the notion of moral obligation, or, in more properly Kantian language, the power of the mind to dictate moral laws for itself. Kant argues that moral obligation cannot come from experience, and cannot be conditioned by it. There are, of course, reasons that fall within the realm of experience that make us do things. If I see someone drowning, my understanding might say, “I should save him because if I don’t I’ll be perceived as cowardly.” But Kant believes that we feel something stronger and more universal when we act morally. We formulate our actions as universal laws. “I must help him, because each person has a duty to help those weaker than himself.”
The source of freedom lies not in our ability to do as we please, but rather in our ability to formulate moral laws for ourselves. This argument, which is Kant’s best-known contribution to philosophy, is explored further in the Groundwork for the Metaphysics of Morals and the Critique of Practical Reason.
Another crucial role played by Reason is that of critique. “Critique,” as Kant understands the term, and as it is meant in the titles of his major works, is the use of Reason to analyze ideas of Reason, to probe them for internal contradictions. Reason is a tool of enlightenment, used to help us better understand what it is we want to know, and whether we can actually get there.
The notion that philosophy’s fundamental goal is not to provide answers, but rather to better understand the questions, as well as to test the soundness of pre-existing ideas, has had a lasting impact on philosophy. Its effects can be felt in thinkers outwardly hostile to Kant’s thinking, like the young Karl Marx, who famously described the task of the philosopher as “the ruthless criticism of everything existing,” and Friedrich Nietzsche, who described his philosophical style as akin to using “a hammer as a tuning fork.”
