Now Kant turns to the question of how the natural sciences are possible. He begins by defining nature. Nature, in Kant’s definition, is the existence of things insofar as they are determined by universal laws. Kant stresses once more that by “things” he does not mean things in themselves, but things as they appear to us.
How is it possible to think something as being governed by a law? It would be impossible to know a priori that something was governed by a law, because that could never be contained in the concept of a thing. We don’t know, say, from a definition of a bicycle as a vehicle with two wheels, that it is subject to the law of gravitation. By things, Kant reminds the reader, he does not mean things themselves, since we can have no knowledge of these, but rather things as they appear to us.
How is it possible to think something as being governed by a law? Kant repeats that this is not possible a priori. Nor is it possible a posteriori. If, like Newton, an apple fell on our head, that would not be proof of gravitation—only proof that the apple fell that one time. Because experience is finite and contingent, we can’t perceive things as necessarily happening.
Nonetheless, physics has demonstrated that thinking things as determined by universal laws is, in fact, possible. But that raises more questions than it answers. Like mathematics, physics has an empirical component. Its concepts are taken from the visible world: motion, inertia, impenetrability. And yet, it yields a priori laws, like that of gravitation. How is this possible?
Here, Kant adds a second definition of nature: nature is all objects in existence taken altogether. The natural sciences are based on a combination of these two definitions of nature: the understanding of all objects in existence as being determined by universal laws. How, then, is it possible to know that all perceivable objects are determined by universal laws?
Kant answers this question by drawing a distinction between “judgments of experience” and “empirical judgments.” Empirical judgments are simply sensory impressions—subjective judgments that make no claim to objectivity, but rather describe the sensations of the person making them—e.g. “This coffee is hot.”
Judgments of experience, however, combine sensory impressions with other sensory impressions using what Kant refers to as pure concepts of the understanding, or means used by the mind to connect subjective impressions like causality, e.g. “This coffee is hot because I saw that it was just brewed.”
Pure concepts of the understanding are means by which subjective, empirical judgments become universally valid, objective judgments. Kant uses the example of air: “the air is warm” is a subjective judgment. We would not object if someone less sensitive to warmth—someone from a tropical island, say—might not find the air so warm. But when we say “all air is elastic,” that is, that it has a changeable shape and will, therefore, fill any volume that contains it—we form this as a hypothetical: if the substance we feel before us is air, then it is elastic. A judgment of this kind is an objective one; when we make it, we expect all other rational people to agree with us.
Kant enumerates the four categories of judgment, by means of which we can judge a given object: according to quantity: universal (all Xs); particular (some Xs); and singular (this X); according to quality: affirmative (Xs are Ys); negative (no Xs are Ys); and infinite (Xs are not-Ys); according to relation: categorical (Xs are Ys); hypothetical (if X then Y), disjunctive (X is either Y or Z); and modality: problematical (Xs are possibly Gs); assertoric (Xs are in fact Ys); and apodictic (Xs must be Ys).
Kant also enumerates twelve pure categories of the understandings: there are those of quality: unity, (one X); plurality (some Xs); totality (all Xs); quality: reality (X exists), negation (X does not exist), and limitation (X is present until Y, and then is absent); relation: inherence and substance (Y is a quality of X); causality (X causes Y); and community (X causes Y and Y causes X).
These categories are called pure because, as Hume correctly observed, they cannot be derived from experience. But, Kant argues, that doesn’t mean that we don’t use them or that they are illegitimate (as Hume thought they were)—they simply originate in the mind.
What the faculty of the understanding does, then, is link subjective impressions using the pure concepts, and combine all of these judgments into a single, unified consciousness of the world. The understanding manages the interplay between our subjective impressions and our objective judgments. For example, I see that there is no train in the station, and I hear an announcement saying that the train is delayed. These are both subjective impressions, and using the pure concept of causality, I combine them into the judgment that the train is not in the station because it is delayed. The laws of natural science are derived from the pure concepts of the understanding.
Kant closes by repeating that, because the laws of natural science are derived from the mind, we cannot say that we know with certainty that they hold for the world beyond our perception. Like space and time, they are forms that the human mind imposes on its perceptions. They are the necessary conditions for us to experience the world as we do. Their universality tempts us to say that they will hold for objects beyond our perception, but we cannot say that with true certainty.
Analysis
Just as the first part of the Prolegomena focused on sense perception, and tried to discern what forms of objective knowledge it offered us (i.e., the pure intuitions of space and time), the second part of the Prolegomena examines what objective knowledge is possible via our understanding. Though ostensibly about the natural sciences, the second section is really a showcase for Kant’s epistemology—the science of determining how we know what we know.
Kant’s major debating partner here is David Hume. Twenty years before the Prolegomena, Hume argued that all knowledge derived from sensory experience, and that all of our abstract certainties were simply the cumulative weight of our these experiences. We don’t know—not for certain—that the sun will rise tomorrow, Hume argued; all we have is the cumulative memory of the sun having risen every single day so far, which furnishes us with what is really just an educated guess: it is very likely that the sun will rise tomorrow. The only way to know for certain if it will rise is to watch it rise each day.
Most significantly, Hume rejected the notion that we could know for certain that one thing caused another: a handclap and its sound, for example. All we know for certain is that the visual impression of two hands coming together and the attendant sound came in very close succession.
Unlike Hume, Kant devoted considerable labor to the sciences, and as a result, he disagrees. Kant says that if Hume were right, science would be impossible. Hume’s major error, Kant believes, is that he conflates two types of judgment. He agrees with Hume that combinations of sensory impressions can never rise to the status of a universally valid judgment—a judgment with which we would expect every agree. What makes that judgment objective is our use of a pure concept of the understanding: we don’t expect the sun to rise tomorrow because we’ve seen it happen before; we know that it rises because the earth orbits the sun. Judgments of experience carry with them an expectation that everyone must agree—Kant believes that Hume fails to recognize this.
The second section also crucially gives us a picture of what the act of thinking means to Kant. The thinking that we do on a day-to-day basis is not of the speculative, imaginative kind. Instead, reflective thought serves to unify all of the sensory impressions we are receiving at any moment into a unified consciousness, a stable picture of the world. The legalistic sense of Kant’s vocabulary (judgment, right) has an important political resonance here: the mind is a kind of judge, or police officer, a government that arranges the clamor of our senses into an orderly whole.
Kant is thereby able to steer philosophy out of the dead end into which he thinks Hume has led it. Because, if all I have are my sensory perceptions, then how can I be sure that other people exist? How can I interact with other people, and be sure I live in the same world that they do? Kant’s answer is that, though sensory impressions differ, all minds are hard-wired to combine them in the same way. The philosopher Hannah Arendt has pointed out that for Kant thinking is therefore something that necessarily happens not in isolation, but among a community of minds, even if this community is only imagined. Kant will explore the implications of this sense of community in his moral, political, and aesthetic philosophies.
In Kant’s time, however, it was far from clear that he had rescued anything from Hume’s skepticism. His first readers, like Johann Feder and Christian Garve, felt that, despite his claims to the contrary, Kant was still an idealist—that he believed that the world was a projection of the human mind. While Kant clearly feels that he has rescued the sciences, the caveat that we can still have no knowledge of things beyond our perception feels like a considerable concession, leaving open the possibility that we exist in, and are at the mercy of, a world of complete and total chaos.
Still more damaging for Kant is his failure to account for the relationship between things in themselves and their appearances to the human mind. If things have appearances, doesn’t that mean that they cause a certain reaction in the mind? That would entail extending the category of causation beyond our perception, to things in themselves—which is precisely what Kant says we cannot do. And yet Kant does not explicitly say that there is no relation between things and their appearances. He simply sidesteps the question as unanswerable.
This splitting of the world into the realm of actuality and the realm of appearance will continue to plague Kant. Philosophers like Fichte, Schelling and Hegel, known as the German Idealists, would use precisely this problem as the starting point of their radical overhaul of the Kantian philosophy by claiming that appearance is reality, and that the structures of the mind and the structures of the world are simply one and the same.
