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Understanding Kant’s Critique of Pure Reason

In 1781, after a decade of introspection, Immanuel Kant published a book so difficult that even leading philosophers struggled to finish it. Kant’s friend and rival Moses Mendelssohn described the Critique of Pure Reason as a “nerve-juice consuming book.” Yet, buried within its more than 800 pages is one of the most influential ideas in the history of thought—an idea that Kant himself compared to the revolution begun by Copernicus in astronomy.

Previously, everyone believed that the Sun revolved around the Earth. Copernicus (1473-1543) turned the tables by asking how it would be if, instead, the Earth revolved around the Sun. Kant believed that human thought itself required a similar reinvention. It had always been assumed that human knowledge must conform to the world, that the human mind was just a passive observer. But what if it was the other way round? What if it was the world that needed to conform to the structures of the human mind?

Why Kant’s Revolution Was Needed

Since Descartes (1596-1650), a debate had been dominating philosophy. On one side stood the rationalists such as Descartes, Spinoza, and Leibniz, who believed that the senses were deceptive and that reason was the only secure source of knowledge. On the other side stood the empiricists, especially Locke and Hume, who argued that the mind starts as a blank slate and that all knowledge is founded in sense experience.

Hume (1711-1776) pushed the empiricist position to its logical conclusion, which is that there are only two kinds of human knowledge: “matters of fact”, which are founded in observation, and “relations of ideas,” such as logic and mathematics, which are true by definition. Anything that is neither, including God, the soul, and free will, belongs to the realm of illusion and speculation.

This scepticism deeply troubled Kant. Hume’s position undermined many of the concepts on which science and social life depend, not least causation. We assume that causes produce effects, but never—according to Hume—observe causality itself: all we do is observe one event regularly following another. Kant fully acknowledged his debt to Hume: “I freely confess: it was the objection [to the principle of cause and effect] of David Hume that first … interrupted my dogmatic slumber…”

The Synthetic à Priori

Kant’s response was to seek a new foundation for knowledge, based on a distinction between different kinds of judgement.

Some statements, such as, “All bachelors are unmarried men,” are analytic, in that their truth is contained within the meanings of the words themselves. Other statements, such as, “The cat is on the mat,” are synthetic, in that they carry new information. Most of the claims that we make are synthetic, because, unless you’re a pedant like me, new information about the world is a lot more useful and interesting than tautologizing.

Kant also distinguished between knowledge that is à priori (acquired independently of sense experience) and knowledge that is à posteriori (acquired from sense experience).

Whereas analytic truths are à priori, synthetic truths are à posteriori. But could some synthetic truths be à priori? If so, we could learn something genuinely new independently of sense experience.

Kant believed that mathematics, geometry, and, indeed, causation are prime examples of synthetic à priori knowledge.

But how could reason alone provide us with new knowledge, independently of sense experience?

Kant’s Categories

Kant’s answer was revolutionary. The human mind is not a passive recipient of information, but actively shapes experience.

The mind possesses built-in structures that organize raw sense data. Among these “categories of understanding” are concepts such as causality, substance, unity, plurality, and necessity. 

Whenever we experience the world, these categories are already at work. They are not learned from experience; rather, they make experience possible in the first place. They package raw sense data into forms that are graspable to the human mind. If, as per Hume, we never observe causality, this is because causality is a feature of the human mind.

The same is true also of space and time, which are not features of the world, but “forms of intuition” by which the human mind organises sense experience. Space is the framework by which we perceive external objects. Time is the framework by which we order events and mental states. 

That space, time, and causation are in the mind does not make them subjective fantasies. Because all humans share these cognitive structures, they provide a common framework for objective experience. Simply put, there is, for a human being, no other way of seeing the world.

The Limits of Metaphysics

So here we have it. The mind does not conform to an independently structured world; rather, the world as we experience it conforms to the structures of the mind. Thus, we can know some things about the world by knowing about our mind. This is what underlies and enables the synthetic à priori. This is, in other words, what makes it possible for reason, working outside of sense experience, to arrive at new knowledge.

Causation and mathematics arise from the very conditions that make human experience possible. But the flipside is that we cannot know what lies outside of human experience. We cannot know how the world is in itself. Unlike the phenomenal world of human experience, this noumenal world is closed to reason, science, and knowledge.

Thus, metaphysical questions about God, the soul, and freedom are outside the scope of human knowledge. Kant may have rescued causation from Hume and the empiricists, but he did not rescue God (although, later, he defended belief in God on moral grounds). He earned the nickname “the All-Crusher” because he seemed to have demolished centuries of metaphysical speculation about God, the soul, and free will, which, he thought, were outside the scope of human knowledge.

The Antinomies of Pure Reason

Whenever reason attempts to venture into the realm of metaphysics, it becomes entangled in contradictions. Kant demonstrated this through four specific “antinomies of pure reason,” that is, contradictions that reason falls into when trying to understand ultimate reality.

Here is my own example of an antinomy of pure reason. After more than two thousand years of debate, there is still no consensus on who made the stronger case: Heraclitus, in arguing that the universe is in constant, perpetual flux (thesis), or Parmenides in arguing that it is in fact a single, static, unchanging, and eternal whole (antithesis). 

Reason, Kant noticed, can construct persuasive arguments on both sides. The resulting conflicts reveal not the failure of reason but the danger of taking it beyond its proper domain. Plato demonstrated something similar in the Parmenides, though less explicitly than Kant.

The Legacy of the Critique of Pure Reason

Unlike the German idealists whom he inspired (Fichte, Schelling, Hegel…), Kant did not go so far as to claim that the mind creates reality, only that the world as it appears to us is structured through the forms and categories of human cognition.

Since Kant’s time, developments in modern physics, particularly non-Euclidean geometry and Einstein’s theory of relativity, have challenged some of Kant’s specific claims about space. But his central insight continues to shape philosophy, psychology, and cognitive science.

We do not simply discover the world. We actively shape the way it appears to us. This was Kant’s Copernican revolution: the insight that knowledge is not merely a mirror of reality but the outcome of a dynamic interaction between the mind and the world.  By redefining the relationship between human beings and experience, Kant transformed philosophy as profoundly as Copernicus transformed astronomy.

Neel Burton is the author of the newly published The German Greeks: German Philosophy and the German Philosophers.

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Is Kant’s Categorical Imperative too rigid?

When obeying those moral laws that we could consistently and rationally will as universal laws, we are following the so-called Categorical Imperative, which might be re-stated as, “Always act such that the maxim of your action can at the same time be upheld as a universal law.”

This is similar to the much older Golden Rule of the Bible and Indian Mahabharata, according to which we should treat others as we would wish to be treated. But whereas the Golden Rule is based on personal desire, which is subjective (I might, for example, be a masochist, or be willing to tolerate some mistreatment), the Categorical Imperative is based on reason, which is objective.

Hypothetical vs Categorical Imperatives

Hypothetical imperatives are practical rules for achieving a desired outcome, for example, “If you want to lose weight, you should watch what you eat.” If you do not desire a particular outcome, you do not need to follow the rules. In this much, hypothetical imperatives are conditional and contingent. In contrast, Categorical Imperatives are universal moral commands that bind everyone regardless of their aims, for example, “Do not lie,” “Do not steal,” “Do not commit suicide.” 

Hypothetical imperatives answer to the lower faculty of desire, which aims at pleasure. Categorical Imperatives answer to the higher faculty of will, which functions rationally and autonomously by following the laws which it legislates for itself, regardless of consequences or personal feelings.

For Kant, true moral actions must be motivated by duty, not some desired outcome. Thus, Kantian ethics are sometimes described as deontological, or duty-based [Greek, deon, “duty”], and contrasted with consequentialism (for example, utilitarianism), which is outcome-based. For Kant, moral systems based on outcomes or desires operate on hypothetical imperatives, not true moral law.

Some Examples of the Categorical Imperative

Kant furnishes some examples to flesh out the bones of the Categorical Imperative. Imagine a person in financial need who borrows money and promises to pay it back, knowing full well that they never will. If this action were universalised, promises of repayment would no longer be believed and the practice of lending would end. 

When we help someone, our action must be motivated by duty if it is to have moral worth. If I help someone from inclination, for example, from sympathy or because it makes me feel good, I am still doing a praiseworthy thing, but my action, being circumstantial rather than principled and reliable, lacks moral worth. Imagine a grocer who always gives the correct change, but only to avoid being caught and losing his reputation. His behaviour, though not blameworthy, is lacking in moral worth. If he knew that he could not possibly get caught, he may start behaving dishonestly. Because his behaviour is prudential and circumstantial rather than born out of duty, it is not categorical. 

For Kant, a paradigm of moral worth is the person who hates life and longs to commit suicide, but stays alive purely out of duty. Because this person has no self-serving inclinations, he is acting purely from duty, rather than mere “conformity to duty.” Similarly, and counterintuitively, a hard-hearted person who has no other motivation than duty has a moral worth “beyond all comparison the highest.”

The Humanity Formulation of the Categorical Imperative

The universalizability formulation is the first formulation the Categorical Imperative. The second formulation is the humanity formulation, or end-in-itself formulation: “Act always treating humanity, in yourself and others, as an end and never merely as a means.”

Like Aristotle, Kant argued that everything that has instrumental value derives this value from the end that it serves. Thus, for anything to have value, there must be some end that has intrinsic worth, that is, some end that is an end-in-itself. For Aristotle, this “supreme good” was happiness, or eudaimonia. For Kant, it was a rational being who could freely determine his or her own ends. In all of nature, man alone is an end-in-itself, and must therefore be treated as such. 

We can only use others (like waiters and cab drivers) as means if we respect their own ends and agency, treating them as rational beings with purposes of their own rather than mere tools for achieving ours. You can employ a servant if you pay and treat them fairly, and the servant wills it because working for you furthers their own ends. Although Kant never applied the humanity formulation to specifically and explicitly condemn the transatlantic slave trade, his moral philosophy provided the framework for later abolitionists.

Benjamin Constant’s Challenge to Kant

In the aftermath of the Reign of Terror (1793-1794), a period during the French Revolution marked by mass executions of perceived enemies, the Swiss writer Benjamin Constant conceived of a thought experiment to undermine Kantian ethics.

Imagine an axe-wielding murderer at your door, asking where your friend, who has taken refuge in your house, is hiding. Although, according to Kant, lying is always wrong, it would be absurd to speak the truth and reveal your friend’s location to the murderer. In this scenario, surely, the duty to protect your friend overrides any duty to tell the truth. What’s more, by intending to commit a grave injustice, the murderer has forfeited any right to the truth.

Kant responded to Constant in his 1797 essay, On a Supposed Right to Tell Lies from Benevolent Motives, and dug in his heels. Even in those circumstances, it would be wrong to lie. The morality of an action is determined by its principles, not its consequences. One could not know whether lying would do more good than harm to one’s friend in hiding. Whereas one would be responsible for the consequences of telling a lie, the consequences of telling the truth would be on the murderer. Moreover, to lie to the murderer would be to treat him as a mere means to an end, denying him the status of a rational being capable of free, reasoned action.

The One Problem with Kantian Ethics—and a Solution

Kant’s rigid application of the Categorical Imperative led him to condemn many actions and behaviours that are no longer generally condemned, such as masturbation and suicide out of world-weariness. He referred to masturbation as an “unnatural vice” on the basis that the natural purpose of sex is procreation. 

The Categorical Imperative is, no doubt, a good rule of thumb, but must admit of exceptions. Exceptions, too, are a matter of judgement and reason—more so even than the rules themselves.

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Kant’s Legacy: Free Speech and Individual Rights

In 1783, two years after the publication of his masterpiece, The Critique of Pure Reason, the 59-year-old Immanuel Kant could finally afford to buy himself a house. A wall had to be knocked down to create a lecture room, since lectures in those days were not held on university premises but in private rooms, often in the lecturer’s lodgings.

A year after purchasing his house, Kant published an essay in response to a question posed by the Revered Johann Zöllner in a German periodical. The reactionary reverend had asked against the liberals: “What is enlightenment? This question, which is almost as important as what is truth, should be answered before one begins to enlighten! And still I have never found it answered!”

Kant’s Famous Essay: What is Enlightenment?

Kant published his response, entitled, Answer to the Question: What is Enlightenment, in his preferred outlet, the Berlin Monthly. The short, thousand-word piece remains widely read, and is especially remembered for its first paragraph:

Enlightenment is man’s emergence from his self-imposed immaturity. Immaturity is the inability to use one’s understanding without guidance from another. This immaturity is self-imposed when its cause lies not in lack of understanding, but in lack of resolve and courage to use it without guidance from another. Sapere aude [Dare to Know! Dare to be Wise!] “Have the courage to use your own understanding!”—that is the motto of enlightenment.

Kant goes on to advance free speech, “the freedom to use reason publicly in all matters”, as the prerequisite of progress and enlightenment, and praises his king, Frederick the Great, “a shining example”, for having granted it to him and other Prussian subjects.

What All Enlightenment Thinkers Had in Common

In 1754, following the success of his First Discourse and his one-act opera, Le Devin du village, Rousseau returned to Geneva and converted back from Catholicism to Calvinism (a form of Protestantism). But in private, he embraced a personal, natural religion, or “religion of the heart,” which, together with his belief in the corrupting influence of civilization, set him apart from other Enlightenment thinkers such as Diderot and Voltaire, who championed reason, progress, and atheism.

For all their deep differences, what united the proponents of the Enlightenment (Aufklärung in German, les Lumières in French), from Hume and Voltaire to Kant and Rousseau, was a faith in reason as the source of knowledge and authority, and a commitment to applying reason to challenge superstition, dogma, and absolute authority and achieve human progress.

The Impact of the Enlightenment on Modern Democracy

Enlightenment concepts and principles such as government by consent, the separation of church and state, and individual rights and freedoms including religious freedom and free speech are at the heart of the modern democracies that, much better than any other system of government, guarantee our security, liberty, and prosperity—and, dare I say, dignity.

In his essay, Idea for a Universal History with a Cosmopolitan Purpose, published in the same year as What is Enlightenment, Kant argues that history is a teleological process (a process with an end or goal) tending to the full expression of human rationality and morality, manifested by the establishment of a perfectly just and stable constitution within states (initially, European states), which come together to form a federation, or league of nations, to secure universal individual freedoms and perpetual peace. In 1795, Kant followed up his essay with a much more substantial book entitled, Toward Perpetual Peace.

Thus, although better known for his metaphysics, epistemology, and ethics, Kant is sometimes regarded as the intellectual father of both the European Union and the United Nations, and, more generally, of the rules-based international order.

It’s fair to say, he merited the house that he bought, which, ironically, was bombed and destroyed in the Second World War.

Neel Burton is the author of the newly published The German Greeks: German Philosophy and the German Philosophers.

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An introduction to Kant’s Categorical Imperative.

In 1755, Immanuel Kant (1724-1804) became a Privatdozent, or non-stipendiary lecturer, making a living by charging students for his lectures and giving private tuition. His students joked that he could cover for the entire philosophy faculty—which at the time included everything except the three “higher” faculties of theology, law, and medicine. He became a star lecturer, reputed for his wit, dry humour, and poker face.

When he lectured, Kant was, according to a contemporary account, “all things to all men.” He stood at a diminutive five foot two (1.57m), with blond hair, blue eyes, and rosy cheeks. Despite his slender build, flat and narrow chest, and “slight corkscrew twist”, he was described as attractive. According to his own account, his flat and narrow chest left “little room for the movement of the heart and lungs”, contributing to his delicate constitution, predisposition to hypochondria, and sedentary, regimented lifestyle.

Kant compensated for his physical frailties by being a sharp dresser. He took inspiration from nature for matching colours. Thus, in middle age, he often wore a yellow waistcoat with a brown tailcoat trimmed with gold braid. In public and in company, he sported a powdered wig, even after it had fallen out of fashion. As late as 1791, his friend and follower Joachim Christian Friedrich Schulz described him as having “the look of a good, honest watchmaker who has gone into retirement”.

From 1766, Kant derived a modest but regular income as sub-librarian of the university. From 1768 to 1777, he rented two rooms in the house of the publisher and bookseller Johann Jakob Kanter. In this period, his student Johann Gottfried Herder described him as “the most urbane fellow in the world”. He moved in the city’s most refined circles and often stayed out into the small hours.

Kant never considered himself quite rich enough to take a wife. He twice considered marriage, first to a “beautiful widow” and, much later, to a “pretty Westphalian girl”, but in each case prevaricated for so long that the ladies ended up marrying otherwise. Although he never married, Kant defended the institution of marriage, which he prosaically defined as “the union of two persons of different sexes for lifelong possession of each other’s sexual attributes”.

Kant’s Categorical Imperative

Kant is famous, among other things, for trying to put morality onto a rational basis, that is, for trying to make morality objective and “categorical” rather than subjective and arbitrary. His core ethical principle is the Categorical Imperative, according to which we should only obey those moral laws that we could consistently and rationally will as universal laws. This might be restated as, “Always act such that the maxim of your action can at the same time be upheld as a universal law.”

The Categorical Imperative is similar to the much older Golden Rule of the Bible and Indian Mahabharata, according to which we should treat others as we would ourselves wish to be treated. But whereas the Golden Rule is based on personal desire, which is subjective (I might, for example, be a masochist, or be willing to tolerate some mistreatment), the Categorical Imperative is based on reason, which is objective.

Hypothetical imperatives are practical rules for achieving a desired outcome, for example, “If you want to lose weight, you should watch what you eat.’ If you do not desire a particular outcome, you do not need to follow the rules. In this much, hypothetical imperatives are conditional and contingent. In contrast, Categorical Imperatives are universal moral commands that bind everyone regardless of their aims, for example, “Do not lie”, “Do not steal”, “Do not commit suicide”.

Hypothetical imperatives answer to the lower faculty of desire, which aims at pleasure. Categorical Imperatives answer to the higher faculty of will, which functions rationally and autonomously by following the laws which it legislates for itself, regardless of consequences or personal feelings. For Kant, true moral actions must be motivated by duty, not some desired outcome. Thus, Kantian ethics are sometimes described as deontological, or duty-based (Greek, deon, “duty”), and contrasted with consequentialism (for example, utilitarianism), which is outcome-based. For Kant, moral systems based on outcomes or desires operate on hypothetical imperatives, not true moral law.

Kant furnishes some examples to flesh out the bones of the Categorical Imperative. Imagine a person in financial need who borrows money and promises to pay it back, knowing full well that they never will. If this action were universalised, promises of repayment would no longer be believed and the practice of lending would end. Kant also points out that abusing the lender in this way reduces a dignified being with ends of his own to a mere means-to-an-end.

The universalizability formulation is only the first formulation the Categorical Imperative. The second formulation is the humanity formulation, or end-in-itself formulation: “Act always treating humanity, in yourself and others, as an end and never merely as a means.” 

When Is It Moral to Have Sex?

Kant’s rigid application of the Categorical Imperative led him to condemn many actions and behaviours that are no longer generally condemned, such as suicide out of world-weariness, homosexuality, and masturbation. He referred to homosexual acts and masturbation as “unnatural vices” on the basis that the natural purpose of sex and the reproductive organs is procreation. Marriage, he argued, is the only morally permissible context for sexual relations. But for those unable or unwilling to wait, sex is preferable to masturbation. Masturbation transgresses the natural order, sex, only the civil order.

What has aged a lot better is Kant’s insight that, during sex, one turns the other into an object of gratification, a means to an end rather than an end-in-himself or end-in-herself. This is only permissible if, in return, one makes oneself into an object of gratification for the benefit of the other. Marriage, on the other hand, is a unique union in which two people mutually surrender their bodies and selves to create a united whole.

For us, the take-home message is: Kant never had sex or even masturbated. 

Jokes aside, Kant grew into, and arguably remains, the supreme moral authority in the Western World. For better or worse, what he thought and how he thought remains part of our mental makeup.