Modern Philosophy: Descartes to Nietzsche

You are reading this on a screen, and you are sure the screen is there. Now ask the old question: how do you know this is not a dream, or a very good simulation? You will not find a knockdown reply, and that is the point. From the 1630s to the 1880s European philosophers kept looking for one, and the way they failed produced the map of the subject. Between the scientific revolution and the age of mass politics, they asked what the mind brings to experience, what justifies our belief in causes, what makes a society free, and what is left of value once a religious foundation is gone.

The usual story calls it a duel: rationalists (reason is the source of knowledge) against empiricists (experience is), settled by Kant. It is a useful map and a bad history. The people on it argued, borrowed and wrote letters, and some of the sharpest letters came from women.

Descartes: how much can you doubt?

René Descartes (1596–1650) wanted certainty to rest on something unshakeable, as geometry seemed to. In the Meditations (1641) he decided to reject anything that could be doubted, in stages. First, the senses sometimes deceive (a distant square tower looks round). Second, I might be dreaming right now, and nothing marks the difference. Third, an all-powerful deceiver might make me wrong even about "two and three make five". The doubt survives only one thing: in order to be deceived, I must exist.

"I think, therefore I am."Descartes, Discourse on Method (1637), part IV; in the Meditations the argument comes in the second meditation

Descartes then argued that the mind is a thinking, non-extended thing, and the body an extended, non-thinking thing: dualism. Push the doubt yourself.

Without JavaScript: doubting the senses removes beliefs about distant things; doubting by dream removes "I am here by the fire" and the existence of the outside world; the evil-deceiver doubt removes arithmetic. "I am thinking" and "I exist" remain.

Two objections shaped the next century. Antoine Arnauld said Descartes argues in a circle: he proves God to guarantee clear ideas, using clear ideas. And in 1643 Princess Elisabeth of Bohemia (1618–1680) asked the question he never answered well: if the mind is not extended, how can it move a body at all? We return to her below.

Spinoza and Leibniz: reason takes charge

Baruch Spinoza (1632–1677), expelled from the Amsterdam Jewish community in 1656 and earning his living grinding lenses, took dualism's problem and removed it. There is one substance, "God or Nature," and mind and body are two ways of describing it. Everything follows by necessity, as in geometry; the Ethics (published after his death, in 1677) is written as propositions and proofs. Freedom is not choosing against necessity but understanding it. His critics called it atheism; his defenders note that it is a very different God from either a person or a mere pile of matter.

Gottfried Wilhelm Leibniz (1646–1716), lawyer, diplomat and co-inventor of the calculus, built on the principle of sufficient reason: nothing is so without a reason. From it he drew the existence of a God who chose "the best of all possible worlds", and a universe of simple, mind-like units, monads. Voltaire's Candide (1759) mocked the optimism; Leibniz's actual claim was that a perfect creator would choose the best total package, evil included. Whether that answers the problem of evil is still argued.

Locke, Berkeley and Hume: all knowledge from experience

John Locke (1632–1704) opened the Essay Concerning Human Understanding (1689) by denying innate ideas: the mind starts as blank paper and experience writes on it. George Berkeley (1685–1753) then argued that if all we know are ideas, nothing needs matter behind them: to be is to be perceived (esse est percipi), kept in being by God's constant perception. (The famous "tree in the quad" limerick is by Ronald Knox, not Berkeley.)

David Hume (1711–1776) pushed hardest. In the Enquiry (1748) he divided all objects of reason into relations of ideas (triangles, arithmetic: denying them is a contradiction) and matters of fact (known only from experience). Then he asked what justifies expecting the future to resemble the past. Not logic, since the sun failing to rise is no contradiction. Not experience, since that would use the very assumption in question. This is the problem of induction. See if you can sort the cases.

Without JavaScript: "7 × 8 = 56" is a relation of ideas; "the sun will rise tomorrow" is a matter of fact and cannot be proved by logic; a claim that only those two kinds count as knowledge is itself neither, an objection often raised.

Hume's other legacies: the self may be a bundle of perceptions with no further owner; "reason is, and ought only to be, the slave of the passions"; and you cannot derive an ought from an is without a step that needs defending. Defenders reply to the induction problem with probabilistic methods (36/06) or by arguing that no justification is needed; no answer has won. Mary Shepherd (1777–1847) is a notable early critic, arguing that we can know by reason that events have causes.

Kant: what the mind brings

Immanuel Kant (1724–1804) said Hume woke him from "dogmatic slumber". His reply, in the Critique of Pure Reason (1781, revised 1787), was a reversal he compared to Copernicus. Instead of assuming that the mind conforms to objects, suppose objects as we experience them conform to the mind's built-in forms: space, time and categories such as cause. Then some things can be known in advance of experience (that every event has a cause), not because the world in itself must be so, but because that is how any experience we can have is structured.

"Thoughts without content are void; intuitions without conceptions, blind."Kant, Critique of Pure Reason, A51/B75, J. M. D. Meiklejohn's translation

Kant's key tool is a grid. A judgement is analytic if the predicate is contained in the subject, and synthetic if it adds something; and it is a priori if known independent of experience, a posteriori if not. The hard question, he thought, is how synthetic a priori judgements are possible, since mathematics and physics seem to contain them.

Without JavaScript: "all bachelors are unmarried" is analytic a priori; "the kettle is boiling" is synthetic a posteriori; Kant held that "7 + 5 = 12" and "every event has a cause" are synthetic a priori.

In ethics, the Groundwork (1785) states the categorical imperative: act only on principles you could will as universal law, and treat humanity never merely as a means. The costs: Kant's "things in themselves" are unknowable, which many found unstable; later logicians (Frege) argued that arithmetic is analytic; and Kant's own writings contain racist and sexist claims, which scholars now debate openly when reading him.

Women and others the canon left out

Women were mostly shut out of universities, so they wrote letters, essays and novels. They were not on the margins of the argument. Elisabeth of Bohemia, exiled princess, corresponded with Descartes from 1643; her mind-body objection was never answered to her satisfaction, and she asked him to explain the passions, which became his last major book. Margaret Cavendish (1623–1673) argued in Observations upon Experimental Philosophy (1666) that matter is self-moving and that peering through instruments is no guarantee of truth, and in the same year published a fantasy, The Blazing World; in 1667 she visited the Royal Society, as a curiosity. Anne Conway (1631–1679) held that all creatures are one substance in different degrees, and Leibniz read her. Émilie du Châtelet (1706–1749) translated and explained Newton and defended the idea that a moving body's energy grows with the square of its speed.

Mary Astell (1666–1731) proposed an educational community for women (1694) and asked, in Some Reflections upon Marriage (1700): "If all Men are born free, how is it that all Women are born Slaves?" Mary Wollstonecraft (1759–1797), in A Vindication of the Rights of Woman (1792), argued that women seem inferior because they are badly educated.

"I do not wish them to have power over men; but over themselves."Wollstonecraft, A Vindication of the Rights of Woman (1792), chapter 4

Outside northern Europe the argument also went on. In Ethiopia, Zera Yacob wrote his rationalist Hatata (inquiry) in 1667, though scholars disagree over whether it was written by him or by a later hand. Anton Wilhelm Amo, born in what is now Ghana around 1703, taught philosophy in German universities. Both are taken up in Islamic, African & Latin American Philosophy (36/11).

Hegel and Mill: history and liberty

G. W. F. Hegel (1770–1831) argued that thought and history are one process: ideas develop by running into their own contradictions and moving beyond them. (The tag "thesis, antithesis, synthesis" is not Hegel's; he rarely used it.) His most read passage, from the Phenomenology of Spirit (1807), is the master and the bondsman. Self-consciousness needs another's recognition; the master wins recognition but depends on the bondsman, who, through work, builds a world and a self. Marx took the idea and turned it towards material conditions. Hegel's critics, from Kierkegaard to Popper, called him obscure or dangerous, and his lectures on world history are rightly criticised as dismissive of Africa and Asia.

John Stuart Mill (1806–1873) is the clearest voice of liberal utilitarianism: actions are right as they promote happiness, and some pleasures count for more than others. In On Liberty (1859) he gave a rule for when society may interfere with an individual.

"The only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others."Mill, On Liberty (1859), chapter 1

He also wrote The Subjection of Women (1869), crediting his wife Harriet Taylor Mill. Apply his rule to a few cases; it is meant to show how the principle works, not to score your morals.

Without JavaScript: staying up all night gaming harms only yourself, so Mill says leave it; driving drunk past a school endangers others, so interference is allowed; an unpopular opinion in print is protected, though inciting a mob is not.

The critics: communitarians say that no act is purely self-regarding; paternalists argue that some interference helps people; and others ask who counts as harmed (does offence?). Mill himself made exceptions, including that one may not sell oneself into slavery.

Nietzsche: after the foundation

Friedrich Nietzsche (1844–1900) wrote in short, provocative pieces rather than systems. In The Gay Science (1882, section 125) a madman announces that "God is dead": not a boast but a warning, that the beliefs on which European morals rested had lost their support, and that nobody had reckoned the cost. In On the Genealogy of Morality (1887) he asked where our values came from and who they served, arguing that moralities are historical products with hidden motives. He held that knowledge is always from some perspective (perspectivism), and in section 341 of The Gay Science he posed the eternal recurrence as a test: could you will your life again, exactly as it is, forever?

Nietzsche was no nationalist or antisemite and mocked both, but after his collapse in 1889 his sister Elisabeth Förster-Nietzsche, who married an antisemitic agitator, controlled his papers and shaped them for nationalist readers, which is how the Nazis found a "Nietzsche" to quote. The critical edition by Giorgio Colli and Mazzino Montinari (from 1967) restored the texts. What remains debated is how to read him: as a moral sceptic, a perfectionist about individuals, or an artist of provocation. Critics say that his contempt for pity and for "the herd" gives dangerous cover to cruelty.

What a graduate just sees

• The strongest case for any belief is the one that survives a strong doubt, and what survives is often small.
• If mind and matter are different kinds of thing, you owe an account of how they interact.
• "The sun will rise" cannot be proved from the past without circular reasoning; induction rests on trust we cannot prove.
• Some of what we think we learn from the world is the shape our minds give it.
• Whether a society may override a person's choices for their own good is a live question, and Mill's "only to prevent harm to others" is where it starts.
• The history of philosophy is a history of letters and objections, and many of the best objections came from people the textbooks omit.
• Values have histories, and asking whose interests they served is a legitimate question.

Where it shows up next

• Philosophy of Science (36/06): Hume's induction is Popper's and Kuhn's starting point.
• Philosophy of Mind (36/05): Elisabeth's question is the mind-body problem, which Spinoza, Cavendish and Conway each answered differently.
• Contemporary Philosophy (36/12): analytic philosophy begins as a rebellion against idealism (Kant and Hegel); the continental line runs through Hegel and Nietzsche.
• In the world: liberal law and free-speech debate (Mill), human-rights language (Kant), the "is it a simulation?" question (Descartes), and the worry about what a machine knows (Hume, Kant).

If you want to go deeper

• 📘 Textbook. Anthony Kenny, The Rise of Modern Philosophy (Oxford University Press, 2006), volume 3 of his New History of Western Philosophy. A common classroom anthology is Roger Ariew and Eric Watkins's Modern Philosophy: An Anthology of Primary Sources (Hackett); check the current edition.
• 📘 Gentler. Roger Scruton, A Short History of Modern Philosophy (Routledge); his views are his own and occasionally argued rather than reported. Peter Adamson's later volumes on the podcast cover women of the period.
• 🌐 Free text. Jonathan Bennett's Early Modern Texts has Descartes, Locke, Berkeley, Hume, Leibniz and others in lightly edited modern English; the Stanford Encyclopedia of Philosophy has entries on every person above, including Elisabeth, Cavendish, Conway, Astell and Shepherd.