The best books are those that tell you what you already know, but didn’t know that you knew.
Short introduction video (90s)
Find out more about the Ataraxia and Ancient Wisdom series—and about me.

The best books are those that tell you what you already know, but didn’t know that you knew.
Short introduction video (90s)
Find out more about the Ataraxia and Ancient Wisdom series—and about me.
What if depression were a blessing as well as a curse? This is a book about how depression can have benefits as well as costs, and how to reap those benefits while making yourself feel better—better, in fact, than ever before.
🏆 Semi-finalist, the BookLife Prize
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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?
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…”
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 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.
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.
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.
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.

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 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.
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 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.
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.
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.

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 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.
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.
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.

In his youth and middle age, the sharply dressed Immanuel Kant (1724-1804) moved in Königsberg’s most refined circles and often stayed out into the small hours. In this period, his student Johann Gottfriend Herder described him as “the most urbane fellow in the world.”
But when Kant turned forty, he underwent a midlife transformation. He quite literally sobered up, abandoning carefree carousing for the disciplined life of the mind. This profound change owed to the early death of a close friend, the dissolute Johann Daniel Funk, together with the making of a new friend, the English merchant Joseph Green, who lived by the clock. Kant essentially adopted Green’s modus vivendi.
For the rest of his productive life, Kant employed a retired solder, Martin Lampe, to wake him up at precisely five-to-five every morning. Lampe would stride into his master’s bedroom and cry out, “Herr Professor, the time is come!” Or in German, Herr Professor, es ist Zeit!
Kant worked at his desk in his nightclothes until his lectures began at seven. At eleven, he would change back into nightclothes and return to his desk. The working day effectively ended at one, when he would take lunch in company in a public inn or restaurant. Lunch would end at three with a round of manufactured jokes—in the belief that laughter was good for digestion.
At three, after lunch, Kant would take his daily constitutional around Königsberg. He would walk alone, from fear that outdoor conversation would lead him to breathe through the mouth. He often wound up at Green’s, with whom he liked to discuss Hume and Rousseau. It is said that the housewives of Königsberg would set their timepieces by the time—seven sharp—at which he left Green’s house.
By automating trivial daily decisions, Kant’s rigid daily routine freed his mind to focus purely on philosophy. It’s because of Kant that I have six of the same shirt.
What kind of jokes did Kant tell after lunch? Some of his jokes have come down to us in his writing. For instance: A man tried to arrange a solemn funeral for a rich relative, but failed in the task: “The more I paid my mourners, the merrier they looked.” A merchant sailed back from India with his fortune, but, in a violent storm, had to throw all his cargo overboard. This upset him so much that his wig turned grey overnight.
Laughter, Kant thought, “is an affect resulting from the sudden transformation of a heightened expectation into nothing.” The mind is deceived into tensing up, but with the deception revealed suddenly relaxes, leading to laughter, which mimics the motions of the mind and is extremely health-giving. Kant never laughed at his own jokes, but always kept a straight face.
In The Critique of Pure Judgement (1790), Kant quoted Voltaire in saying that Heaven had given us two comforts against life’s hardships, hope and sleep—before suggesting that Voltaire “could have added laughter.”
In 1786, Joseph Green died, deeply affecting Kant, who, thereafter, became a lot more housebound.
After a suitable period of mourning, Kant recruited a female cook and began hosting protracted lunches aimed at stimulating the play of thoughts. To this end, he gathered guests from diverse backgrounds and perspectives, believing that the ideal number of guests lay somewhere in between that of the Graces and the Muses (three and nine). All topics great and small were on the table. Philosophy was allowed but not dogmatism, for fear that it would interrupt the convivial flow of ideas.
As well as wine, Kant had a taste for Königsberger Klopse (meatballs in a creamy white sauce with capers), Teltow turnips (an heirloom turnip from the Berlin-Brandenburg region), roast beef, cod, and, as a condiment for the above, English mustard, which he mixed himself.
Joseph Green was the closest friend that Kant ever had. Green had inspired Kant’s routine, and became his philosophical sounding board. Kant allegedly discussed every single sentence of the Critique of Pure Reason with Green before publishing it in 1781.
In the Metaphysics of Morals (1797), Kant ended his discussion of character traits just like, in the Nicomachean Ethics, Aristotle ended his discussion of the same: with an analysis of friendship.
In the Nicomachean Ethics, Aristotle made much of the idea of “perfect friendship”, which, he thought, is only possible between men of reason and virtue. He famously described a perfect friend as “another self”—later paraphrased by Diogenes Laertius as “a single soul dwelling in two bodies.”
Kant deemed “perfect friendship” an ideal that in practice cannot be attained. But in striving for it, we might nonetheless arrive at “moral friendship,” in which two persons feel able to disclose their secret thoughts and feelings to each other. Moral friendship requires a savant mélange of love and respect, love for bringing two people together, and respect for not driving them apart by infringing upon their dignity and autonomy.
From his own experience, Kant came to believe that most people cannot develop their true character until middle age, when they might undergo a “rebirth”. At twenty, we are no more than the product of our upbringing and environment. At thirty, we are still reliant on the judgement and approval of others. Only at forty are we confident enough (or perhaps tired enough) to become who we truly are.
Neel Burton is the author of the newly published The German Greeks: German Philosophy and the German Philosophers.
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