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Book Summary: Critique of Pure Reason – A groundbreaking and influential philosophy classic about the limits of human reason

The Critique of Pure Reason (1781) is one of the most groundbreaking, revolutionary, and influential books in the history of Western philosophy. Pointing out the limits of human reason, it argues that we can have knowledge about the world as we experience it, but we can never know anything about the ultimate nature of reality.


Philosophy, History, Criticism, Rationalist Philosophy, Free Will and Determinism Philosophy, Epistemology Philosophy

Introduction: Get a handle on some of the most complicated ideas in the history of Western philosophy.

What is the nature of space and time? Is the world governed by the law of cause and effect – and if so, why? These are just two of the fascinating questions that Kant raises in the Critique of Pure Reason. His answers are thought-provoking, revolutionary, and even downright mind-blowing.

Unfortunately, they’re also buried in 856 pages of some of the most impenetrable prose ever written. Kant himself described it as “dry, obscure, opposed to all ordinary notions, and moreover long-winded.” Even Kant scholars are unsure of how to understand the Critique’s incredibly complicated arguments, and they’ve put forward many competing interpretations.

In light of these facts, these summaries can only present an interpretation of some of Kant’s main ideas, skipping over many technical details. Fortunately, most of these details are of interest only to scholars. For the rest of us, the gist of Kant’s ideas provide more than enough food for thought.

[Book Summary] Critique of Pure Reason: A groundbreaking and influential philosophy classic about the limits of human reason

In these summaries, you’ll learn about

  • the surprising nature of space and time;
  • the shocking truth behind the law of causality; and
  • the sobering lesson that reason must learn about its own limitations.

Before they build a metaphysical system, philosophers must assess the origin and nature of our minds’ mental materials.

Imagine you’re a builder living in medieval times. One day, the king summons you to a construction site. He points at a massive pile of building materials and says, “I want you to turn this into a tower that reaches heaven – or as close as possible, at least.” What do you do?

Well, you should begin by inquiring into the materials. What are they made of? How strong are they? By answering these questions, you’ll be able to tell how tall you can build the tower. The alternative is to just start building and hope for the best – a recipe for disaster if you end up going higher than the materials can bear.

The same argument applies to philosophers when they want to build a metaphysical system.

The key message here is: Before they build a metaphysical system, philosophers must assess the origin and nature of our minds’ mental materials.

Metaphysics is the branch of philosophy that tries to elevate our knowledge of the world into the loftiest realms of human inquiry. Using the abstract concepts and logical principles of reason, it attempts to go beyond the empirical evidence of the natural sciences and grasp the ultimate nature of reality.

Consider time, for instance. Does it have a beginning? Or does it stretch back into eternity? These are examples of metaphysical questions. Answer a bunch of them, tie your answers together into a coherent body of thought, and you have a metaphysical system.

Since the days of ancient Greece, many philosophers have attempted to build various metaphysical systems. But prior to the Critique of Pure Reason, most of them tried to do so without first inquiring into the origins and nature of our minds’ mental materials. They just took the concepts and logical principles they had at hand and started building with them.

But are those materials actually fit for the task of metaphysics? And if so, how high do they allow us to build our metaphysical towers, so to speak? If the answer is “all the way to heaven,” then great – build away. But if it’s “not very far,” then we should stay closer to the earth – contenting ourselves with the more mundane knowledge of the sciences, while leaving the more esoteric stuff to religion.

Either way, we need to know the answer in advance. Otherwise, we’ll be in danger of building a tower that will crumble beneath our feet.

To avoid the danger of metaphysical dogmatism, philosophers must conduct a critique of pure reason.

Alright, so our metaphysical tower might come crashing down if we don’t investigate the mental materials we’re going to build it out of. But so what? After all, it’s not an actual, physical tower. It’s just a bunch of ideas. No one’s going to get hurt if it falls. So why not just build and see what happens?

Because that’s not a very philosophical approach. In fact, it would be the opposite of philosophy, which is dogmatism.

The key message here is: To avoid the danger of metaphysical dogmatism, philosophers must conduct a critique of pure reason.

Among other things, philosophy is about submitting our beliefs to critical scrutiny. Let’s say you believe that you have free will. Okay, why do you believe that? Maybe it’s because you think people need to have free will to be morally responsible. Alright, but why do you believe that?

The more we uncover the underlying premises of our beliefs and challenge them to see if they withstand scrutiny, the more we’re doing philosophy. In contrast, the more we take our premises for granted, the more we’re engaging in dogmatism – the archenemy of philosophy.

Now, if we just jump straight into building a metaphysical system without first examining the mental materials we’re going to build it from, we’re taking for granted the premise that our materials are fit for the task. Maybe they are; maybe they aren’t – but either way, we don’t know, and our assumption is therefore unjustified. We’re thus being dogmatic about our ability to do metaphysics.

To avoid dogmatism, we need to critically examine this ability. Do we even have it? And if so, where does it come from? Well, it can’t come from our senses, because these can only provide us with empirical knowledge about the physical world, not metaphysical knowledge, which goes beyond the empirical, physical domains of science. So it would have to come purely from our ability to engage in reason – or pure reason, in short.

Thus, to avoid dogmatism, we must submit our capacity for pure reason to critical scrutiny. Is it capable of granting us metaphysical knowledge? If so, how and to what extent? We could call this type of critical project a critique. And we could therefore say that we need to engage in a critique of pure reason.

By failing to avoid the danger of dogmatism, philosophy encourages the spread of skepticism.

If you’re a philosopher, dogmatism is one of the worst accusations imaginable. But if you’re just a normal person, you might shrug your shoulders. So what if we go on metaphysical flights of fancy? Again, we’re just dealing with abstract ideas about esoteric subjects here, so what’s the big deal?

The problem is that dogmatism can empower another enemy of philosophy: skepticism. And this one doesn’t just endanger philosophy, but all human knowledge in general.

The key message here is: By failing to avoid the danger of dogmatism, philosophy encourages the spread of skepticism.

It’s easy to feel skeptical about metaphysics. After all, it doesn’t seem to make progress the way other disciplines do. With the empirical sciences, we can see a clear evolution from the ancient Greeks to modern times. Meanwhile, philosophers are still arguing about many of the same things Plato and Aristotle argued about thousands of years ago.

In retrospect, it’s easy to see why there’s been so much room for disagreement. Without having engaged in a critique of pure reason, philosophers were free to dogmatically advance just about any argument they wanted to make.

The predictable result? An endless battle of contradictory claims. There must be some sort of God who created the universe; otherwise, it could not exist, argues one philosopher. Nonsense, says another; the universe could have created itself.

With so much persistent disagreement about even the most basic metaphysical issues, it’s tempting to shrug our shoulders and say, “Well, I guess nothing can be known about metaphysics.” But remember, metaphysics is the domain of pure reason. Putting aside mathematics, which we’ll come back to later, every other discipline of knowledge relies on the empirical evidence provided by the senses. If it’s attainable, metaphysical knowledge could only be accessed through pure reason.

But if pure reason can’t make heads or tails of the very thing it would seem most well-suited for, what good is it? And if our highest faculty is useless for knowing the most fundamental aspects of reality, how could we trust our lower faculties about the technical details? Perhaps our senses deceive us. If so, our apparently incompetent reason would seem unable to come to our rescue.

And in that case, we might be tempted to shrug our shoulders one last time and say, “Well, I guess we can’t really know anything about anything then.”

Both religion and science depend on metaphysical concepts, making skepticism a danger to both of them.

For those of us living in the age of “fake news” and “alternative facts,” the problem of skepticism is something we’re already familiar with. In eighteenth-century Europe, when Kant was writing, it also felt like a danger, but for different reasons.

Science was on the rise, and mounting skepticism about metaphysics also meant mounting skepticism about religion. But that skepticism cut both ways, and it was starting to undermine the foundation of science as well.

The message here is: Both religion and science depend on metaphysical concepts, making skepticism a danger to both of them.

Let’s start with religion. Many religious beliefs depend on metaphysical ideas. They’re about things like God and the soul, which are supposed to exist in some sort of immaterial, beyond-the-physical – or, in other words, metaphysical – realm of being.

By definition, that realm is beyond the reach of the senses. We’ll never see God in a telescope, a soul in a microscope, or any other metaphysical entity by any other means of empirical observation. But if reason can’t know anything about them either, all such beliefs would be unsupported and unsupportable.

As far as knowledge goes, that would seem to leave us with only the cold, hard facts of science and the laws they establish about the physical world – above all, the law of causality. This law is at the core of science. It dictates that for every given event, there must be another event that makes it happen. All phenomena are therefore just a matter of cause and effect, and science’s task is to figure out the details of nature’s various causal mechanisms. Gravity, the conservation of matter, you name it – all these particular laws of science presuppose the general law of causality.

But here’s the thing: our notion of causality is itself a metaphysical concept. It’s an idea about an aspect of reality that we can never directly observe. Nonetheless, we assume it plays an essential role in structuring how the universe works.

Here, however, the Scottish philosopher David Hume made a point that greatly influenced Kant’s thinking. It goes like this: if we just focus on the evidence provided by our senses, all we see are various things that happen in conjunction with each other. First you turn on the stove. Then the water boils. If you observe this happening many times, you can say that one thing tends to follow another, which establishes a pattern describing those events.

But that’s not the same as saying one thing must follow another, which establishes a law governing those events. You can assume the pattern will continue to hold true and act like a law. But based on the evidence of the senses alone, that’s an unwarranted assumption.

If reason cannot provide us with a priori knowledge, then it cannot secure our knowledge of mathematics either.

From the idea of God to the notion of causality, all of the metaphysical concepts of religion and science are now in jeopardy. But that’s not all. The same basic argument that applies to metaphysical concepts also applies to something called a priori knowledge.

What’s that? Well, the short answer is that it’s a pretty big deal. Mathematical knowledge depends on it, so now math is in trouble too.

The key message here is: If reason cannot provide us with a priori knowledge, then it cannot secure our knowledge of mathematics either.

If we say we know something a priori, that’s a fancy Latin way of saying we know it’s true independently of our experience. For instance, consider the equation 7 + 5 = 12. This simple bit of arithmetic is an example of a priori knowledge. But to understand why, we’re going to need to unpack it a bit.

As innocuous as it might seem, an equation like 7 + 5 = 12 is actually making a surprisingly bold claim about reality. In stating it as a fact, we’re not just meekly suggesting that 7 + 5 seems to equal 12, tends to equal 12, or has equaled 12 in all cases we’re aware of.

No, we’re audaciously declaring that 7 + 5 must equal 12, no matter when or where we’re adding the numbers in question. It doesn’t matter if we’re adding 7 + 5 sausage dogs in eighteenth-century Germany, little green aliens on twenty-third century Mars, or flying pink elephants in our present-day imagination. The sum of 7 + 5 must always and everywhere equal 12.

In other words, the equation is necessarily and universally true. But as we’ve seen with causality, experience never furnishes us with knowledge that one thing must follow another. It just shows us examples of one thing tending to follow another. From these tendencies, we can only deduce patterns, not laws.

As a rule, then, we can say that if we know something is necessarily and universally true, our knowledge cannot derive from experience. By definition, that means it must be a priori. And if it doesn’t come from experience, then that leaves us with two options: either it comes from our ability to reason, in which case it could be secure. Or it’s just a figment of our imagination, in which case it wouldn’t be knowledge at all.

But how could it come from reason? That is the question.

A priori knowledge isn’t innate knowledge; it’s knowledge that the mind produces through its own internal mechanisms.

We’ve come a long way in our journey, so let’s recap: If it’s possible for us to gain metaphysical knowledge, it couldn’t be empirical knowledge derived from experience. It would have to be a priori knowledge derived from reason. So now we’re looking for a way of gaining that.

But is a priori knowledge even possible? If you stop to think about it, the idea of it seems counterintuitive and even downright preposterous. What’s it implying? That we know things like mathematical equations before we have any experience of the world? That we’re somehow born with this knowledge?

Not quite. There’s a major difference between innate knowledge and a priori knowledge, and it’s crucial to understanding the arguments that follow.

The key message here is: a priori knowledge isn’t innate knowledge; it’s knowledge that the mind produces through its own internal mechanisms.

Let’s go back to our equation: 7 + 5 = 12. At some point in our lives, we were taught this equation, and we developed our knowledge of math over many years of education. In other words, we acquired our knowledge in the context of some sort of experience, such as being at school. In this and all other cases, we can therefore say that our experience chronologically precedes our knowledge.

But that doesn’t necessarily mean that our knowledge arises from experience in a causal sense. To see why, think of consciousness as the outcome of forces meeting each other from two directions. On one side, we have the sense data we receive from our sense organs – sounds, smells, images, and things like that. On the other side, we have the internal mechanisms through which our mind processes that data – producing our perceptions, concepts, judgments, and so forth. Get the two sides interacting, and you have consciousness.

In asking whether a piece of knowledge is a priori, we’re essentially asking: which side of the equation does it come from? Our sense data, our minds’ internal mechanisms, or a bit of both? If the answer is that it’s just a product of our minds’ internal mechanisms, then that knowledge would be a priori. While it would still be chronologically preceded by experience, it wouldn’t arise from or depend on that experience. It would be a part of the contribution that our mind itself brings to the table of consciousness.

The mind can be divided into three main faculties – sensibility, understanding, and reason.

Here’s another way of thinking about a priori knowledge: sure, the mind’s mental machinery wouldn’t begin turning its wheels if it didn’t have sense data to process. But at the same time, that data wouldn’t be processed if the machinery wasn’t already there to work on it.

For example, when you were a child, maybe you needed the formula 7 + 5 = 12 written on a chalkboard in order to bring it to your attention. But still, your mind’s mental machinery needed to already have a way of understanding it in order to acquire knowledge of this universal and necessary truth.

If we paid attention to that machinery and its internal operations, we could, in theory, develop a priori knowledge. Our minds would just need to turn inwards and come to know themselves. No experience necessary.

To pursue this hypothesis, we’ll need to roll up our sleeves and investigate the mind’s mental machinery. What are its mechanisms and how do they work? Let’s take a look.

The key message here is: The mind can be divided into three main faculties – sensibility, understanding, and reason.

As its name suggests, sensibility is our ability to have sensations – things like sound, heat, texture, and so on. Let’s say you’re looking at a house. Your visual image ultimately consists of various sensations of color and shape. These sensations, in turn, are the result of your senses being affected by external objects. If light strikes your eyes in a certain manner, you have a sensation of red.

Now, isolated sensations of things like color and shape aren’t that useful in and of themselves. We need to be able to turn this raw data into meaningful information that we can act upon. And that brings us to our next mental faculty: understanding. This is our minds’ ability to form concepts out of sense data, which then enable us to make judgments about the world.

For example, through various experiences, you can eventually form concepts about dogs and happiness. You can then combine these concepts to form a judgment, which asserts a logical relationship between two or more things. For instance, “if a dog is wagging its tail, it is happy.” This judgment asserts an if-then logical relationship between the concepts at issue.

Finally, if you link multiple propositional judgments together, you have a logical syllogism – a chain of reasoning. Here’s where the faculty of reason comes into the picture. To continue with the previous example, you could reason along the following lines: “If a dog is wagging its tail, it is happy. This dog is wagging its tail. Therefore, it is happy.”

So, those are the broad outlines of how the mind works. We’ll look at more details in the summaries ahead.

To organize sense data into meaningful information, the mind needs predefined ways of structuring them.

Some of the ideas we’re about to explore get a little complicated, but the basic one underlying them is pretty simple. It’s the idea that the mind isn’t just a passive recipient of sense data, as if it were a blank slate. Instead, it plays an active role in shaping that data into meaningful information about the world.

How exactly does it do that? Well, that’s where things get complicated – especially when we delve into the details. But again, the basic ideas are fairly straightforward.

The key message here is: To organize sense data into meaningful information, the mind needs predefined ways of structuring them.

In and of themselves, our sense data are just a chaotic jumble of colors, shapes, sounds, smells, textures, and so forth. But we never experience them in that way; by the time we become aware of them, they’re already organized for us. And there’s a reason for that: our minds have already structured them. However, this requires the mind to have certain preset templates or procedures for putting that data into order.

To see why, let’s go back to the example of a house. As you look at it, you see various colors and shapes corresponding to the roof, windows, mailbox, and so forth. But they don’t appear as a jumble. Instead, you see that the roof is above the windows, while the mailbox is next to the door. In other words, you see these images occupying different positions in relation to each other.

These spatial relationships take the sensory content of your experience and give it a form. In other words, they describe the structure of that content – the way the various components of it are organized in relation to each other. If the windows were on top of the roof, well, you’d have a pretty strange house – and the form of your experience’s sensory content would be quite different.

Now, all of these spatial positions and relationships presuppose one obvious thing: space itself. In order to be above, below, next to, behind, in front of, or anything else in relation to each other, things need to be placed together in a shared framework of space. And thus, to be able to perceive any object as being in any spatial position or relationship whatsoever, the mind must already have a framework of space to put it in, prior to encountering the object.

Time and space are pure forms of sensibility, which provide the mind with templates for organizing sense data.

Imagine if your mind didn’t have a framework of space in which it could organize the content of your sensory experience. You’d see a patch of color; let’s say blue. And then – well, that’s it. You couldn’t see another patch at the same time, because there wouldn’t be any space in which they could be co-present. Space is what allows these sensations to be simultaneously part of the same field of vision.

Given how fundamental space is to our minds’ ability to organize our sense data, we can call it a pure form of sensibility. That’s a shorthand way of saying that as a framework for organizing our sensations, space exists in the mind on a “pure,” a priori basis. In other words, the mind already has it ready at hand, prior to experience. It’s like a preprogrammed template the mind can use for structuring its data.

Beside space, there’s only one other pure form of sensibility: time.

The key message here is: Time and space are pure forms of sensibility, which provide the mind with templates for organizing sense data.

Let’s go back to our example of the house. As you’re looking at it, you don’t just see the roof above the door (a spatial relationship). Depending on how you’re looking at it, you’ll either see one of the features before you see the other, at two different moments – or you’ll see them together, at the same moment. Either way, there’s an underlying temporal relationship between your visual sensations. You experience them either simultaneously or sequentially. And the same goes for all of your other sensations; within your overall experience of reality, all of them occur either simultaneously or sequentially in relation to each other.

These temporal relationships are essential aspects of the way your experience of reality is structured. If they all got mixed up, you’d end up having a very different experience. You’d see some pretty bizarre things, like people aging in reverse and objects being in two places at the same time!

But in order for your sensations to have various temporal relationships with each other, there needs to be an underlying continuum of time in which they can take place. Time is therefore the overall framework that makes them possible. And in order to apply that framework to your sensations, your mind needs to have it ready at hand, prior to experience.

Like space, time is therefore another pure form of sensibility. It’s a fundamental aspect of the structure of our sensory experience. Everything we experience takes place within space and time, and thus presupposes them. They therefore not only precede experience; they make our experience possible.

Our minds also contain templates for understanding and reasoning about the world.

By looking at the formal elements of our minds’ sensory content, we were able to identify space and time as the pure forms of sensibility. But sensibility is just one component of the mind; what about understanding and reason?

Remember, these are the mental faculties that enable us to further organize our sense data into concepts, judgments, and logical syllogisms. If we focus on their formal elements, we’ll be able to identify the pure forms of understanding and reason as well.

The key message here is: Our minds also contain templates for understanding and reasoning about the world.

Let’s start with an example of a judgment: “if something is left in the sunlight, it will eventually get warm.” Now, if we ignore the content of this judgment and just focus on its formal structure, we end up with something we could symbolize as follows: “if X, then Y.”

On a formal level, it doesn’t matter what we put into this formula; whether it’s atoms or unicorns, the same basic logical relationship applies to all of them. “If X, then Y” provides your mind with a basic template to follow. It’s like a recipe that says, “Hey, mind, here’s one way you can connect things together.”

In more sophisticated language, we can call this a logical function of understanding. “If X, then Y” is called a hypothetical function, since it asserts a hypothesis about X. Here’s another example: “X is either Y or Z.” This is called a disjunctive function, since it asserts a disjunction – an either/or possibility between X being Y or Z.

With these logical functions in hand, we can then form logical syllogisms. These also have underlying forms, which we can call principles of reasoning. For example, consider the following syllogism: “An animal is either alive or dead. This animal is not alive. Therefore, it is dead.” This takes the form of: “X is either Y or Z. X is not Y. Therefore, it’s Z.”

These logical functions and reasoning principles are some of the most basic ways the mind can join ideas together. And because they’re so basic, the mind must come pre-equipped with them. Otherwise, how could it ever even begin to connect ideas together? It would have to start connecting them before it had any way of connecting them, which is impossible. To get started, it needs some sort of preset templates to follow.

The mind can use its templates to acquire a priori knowledge and concepts.

Our journey has now taken us far. We’ve gone through the principles of reasoning, the logical functions of understanding, and the pure forms of sensibility – also known as space and time. We’ve skipped some details along the way. For example, Kant identifies 12 logical functions and three principles of reason in total. But the ones we’ve covered will do for our purposes.

We’re now ready to return to our question: How is a priori knowledge possible?

The key message here is: The mind can use its templates to acquire a priori knowledge and concepts.

Let’s start with space and time. By analyzing these pure forms of sensibility, we can gain a priori knowledge of mathematics. For example, consider geometry. What is a geometrical figure, such as a circle? It’s basically a form of a possible object in space. It expresses a property of that space – namely, what happens when you take a line and move it around a fixed point.

Study space carefully, and you’ll learn the truths of geometry. But you don’t need to rely on experience to do this. Remember: space is a form of sensibility that exists inside your mind, prior to experience. By studying it, your mind is essentially studying itself – gaining knowledge about one of the ways it structures its experience of the world. This knowledge is therefore a priori.

Now, let’s shift gears to the faculty of understanding and go back to one of our forms of judgement: the hypothetical logical function “if X, then Y.” Here’s another way of putting that: If one thing happens, another thing must happen. Sound familiar? It’s our old friend, the concept of causality!

By thinking about their own logical functions, our minds can form many of the traditional concepts of metaphysics. We can call these concepts the categories of understanding, and there are 12 of them in total, corresponding to the 12 logical functions. In addition to causality, they include the concepts of unity, plurality, existence, and possibility.

But the details aren’t important to understanding the basic point, which is this: these concepts are the result of the mind turning inward and paying attention to itself. Each of them describes a way we make judgments about the world – a way we connect things together in our minds.

As such, they’re pure a priori concepts. We don’t need any experience to form them.

As far as we can know, the categories of understanding only reflect our experience of reality, not reality in itself.

Pretty clear so far, right? Sit tight, the argument goes even further. Not only do we not need any experience to form the concepts that make up the categories of understanding; these concepts help to make our experience possible in the first place.

That’s because conscious experience isn’t just a bunch of sensations in time and space. It’s a combination of those sensations and the thoughts we have about them. These thoughts, in turn, are a matter of making connections between the various sensations and concepts in our minds. For example, you don’t just see a round object; you see it as a bowling ball. Your mind makes a connection between the image – round object, and the concept – bowling ball.

The categories of understanding are the most fundamental way of making such connections. We couldn’t make any connections and therefore couldn’t have any experience without them. From this fact, we can draw a far-reaching conclusion.

The key message here is: As far as we can know, the categories of understanding only reflect our experience of reality, not reality in itself.

Imagine you’re looking at a bowling ball lying on a pillow, creating a depression underneath it. Using the category of causality, your mind connects these two phenomena together in terms of a causal relationship: the bowling ball causes the depression.

But notice the words we’re using here: your mind connects the two things together. In other words, causality is something your mind constructs; it’s something your mind inserts into its experience of reality. It’s your mind’s way of understanding the world. Things just don’t make sense to it until it’s put them into some sort of causal order.

But that means there’s an obvious reason your mind sees causality everywhere it looks: it’s always interpreting things that way! In fact, it must interpret them that way; it’s one of the basic ways in which it’s been programmed to understand the world.

The fact that the mind must interpret things this way explains why causality appears to us as a law. It is a law – a law of the way our minds experience reality. To experience reality, the mind must be able to connect things together in various ways – integrating them into one unified, coherent experience. Linking things in terms of cause and effect is one of the basic ways it does this.

We can thus say with certainty that reality really does exhibit a law of causality – but only insofar as we experience it. As for reality in itself, that we cannot say.

And the same argument applies to all of the other categories.

We cannot know anything about reality in itself – even whether it exists in space and time.

If you’ve made it this far, the hard work of following these arguments is about to pay off. We’re now arriving at one of the most infamous, revolutionary, and mind-blowing ideas in the history of Western philosophy.

Ready? Here we go. Three, two, one…

The key message here is: We cannot know anything about reality in itself – even whether it exists in space and time.

To see why, imagine if space did exist in external reality exactly as we ordinarily think of it. It has three dimensions: width, height, and depth. Alright, now imagine if your mind had a different form of spatial sensibility – one that somehow transformed that three-dimensional reality into something with two, five, or ten dimensions. In that case, you’d perceive the world as having two, five, or ten dimensions. Or imagine if you didn’t have any form of spatial sensibility whatsoever. Then you wouldn’t perceive any dimensions at all.

In other words, even if external reality exists in three-dimensional space, our minds would still have to put their sensations into a three-dimensional spatial framework to perceive them as such. And to do that, they would need to have that framework ready at hand before any sensations got filtered through it. If our minds had a different framework, they’d get filtered differently, and we’d therefore perceive them differently as well.

The same goes for time – and here comes the kicker. If we wanted to confirm that time and space were also aspects of external reality in itself, and not just our internal experience, where would we look? Our sensations, of course – ultimately they’re the only evidence we have of that reality. But by the time they reach our conscious awareness, we’ve already filtered them through our mental frameworks of time and space, along with the categories of our understanding.

So, of course we’ll see time, space, and things like causality in what we perceive – we ourselves put them there! But in doing so, we’ve essentially tainted our evidence. If we look at that evidence and conclude from it that external reality itself has three dimensions, exists in time, or has causality, we’d be like someone wearing red-tinted sunglasses concluding that the world itself is tinted red.

Reason should not venture into metaphysical speculation about the nature of reality in itself.

So wait, are we saying that time, space, and causality don’t exist in external reality – that they exist only in our minds? Not necessarily. This is a very thorny issue within interpretations of Kant’s thought, so let’s just go with the safest one here.

The safest interpretation is that we just don’t know. Maybe time, space, and causality do exist in external reality, and our minds accurately reflect them. Or maybe they don’t, and they’re just the templates of our sensibility and understanding. The point is that we simply cannot tell one way or the other. Wherever we look, we’ll always see things in terms of the frameworks of time, space, and causality that our minds have already imposed on our sense data.

That’s the most modest version of the argument. But even in this form, it has far-reaching implications.

The key message here is: Reason should not venture into metaphysical speculation about the nature of reality in itself.

Thanks to the forms of sensibility and the categories of understanding, we can know a lot about the world as we experience it. For instance, the truths of geometry really do describe the nature of space in that world. The law of causality really does describe the way that events unfold in that world. And the same thing goes for all of the sub-varieties of that law, such as the laws of motion. They too describe the way things happen in that world.

But the key words here are “in that world.” As far as we know, these truths and laws only apply to the world as we experience it – the world of phenomena, as Kant calls it. As for what reality is like in and of itself – the world of noumena – that we can never know. After all, we can only know reality insofar as we experience it. And by the time we’ve experienced it, our minds have already shaped the mental materials of our sensibility and understanding in so many fundamental ways that we’re no longer in a position to say anything about reality in itself.

But those materials are all that reason has to work with. And that leaves reason unable to know the ultimate nature of reality in itself. It can help us to think about the physical world of phenomena, but it can only speculate about the metaphysical nature of noumena. If it tries to go beyond its limits and enter that mysterious realm, it can only spin arguments; it cannot gain knowledge.

Final Summary

The key message in these summaries:

The human mind plays an active role in shaping its own experience and understanding of reality. To play that role, it needs templates for structuring sense data, concepts, and judgments into a coherent picture of the world. Because these templates are internal to the mind, and because we can never perceive or understand reality without them, we cannot know if they reflect the nature of reality as it exists in itself, independently of our minds. This leaves open the possibility that reality in itself is very different than our experience of it. It also makes metaphysical theories of that reality a matter of pure speculation. We should therefore stick to scientifically understanding the empirical realm, while leaving the metaphysical realm to religion.

About the author

Immanuel Kant (1724-1804) was one of the most influential philosophers of all time. His comprehensive and profound thinking on aesthetics, ethics and knowledge has had an immense impact on all subsequent philosophy. Marcus Weigelt’s lucid reworking of Max Müller’s classic translation makes the critique accessible to a new generation of readers, while his informative introduction places the work in context and elucidates Kant’s main arguments.

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