The seventeenth century inherited Aristotle’s physics and a Europe half-destroyed by religious war, and it wanted a certainty that did not depend on any church or ancient authority. René Descartes, a soldier and mathematician, spent a winter’s day in 1619 shut in a heated room in Germany, resolving to doubt everything he could and see what survived. What survived was the doubter. “I think, therefore I am” appeared in print in 1637.
The method was Euclid’s: start from what cannot be denied and build by deduction. Baruch Spinoza, a lens-grinder expelled from the Amsterdam Jewish community at twenty-three, took the same method and arrived at a universe with one substance, in which God and Nature are the same thing. Gottfried Leibniz, who invented calculus in his spare time, took it and arrived at infinitely many substances, each a mirror of the whole. Same tools, three worlds.
Some truths are known by reason alone. Before any experience, the mind can grasp that a triangle has three sides and that nothing comes from nothing.
Doubt is a tool. Strip away everything that could be false; whatever is left is bedrock. Descartes found one stone and built on it.
Mathematics is the model. Knowledge should look like geometry: definitions, axioms, proofs. Spinoza’s Ethics is literally written that way.
What the world is made of. For Descartes, two substances, mind and matter. For Spinoza, one, which he called God or Nature. For Leibniz, infinitely many monads, each perceiving the whole.
Nothing without a reason. Leibniz’s principle of sufficient reason: for every fact there is a reason why it is so and not otherwise, even if we never learn it.
Princess Elisabeth of Bohemia wrote to Descartes in 1643 to ask how a mind with no extension could move a body that had it. He never answered well, and the six-year correspondence is the best criticism his system ever received. Émilie du Châtelet’s Foundations of Physics (1740) was the clearest Leibnizian physics of the century, and her translation of Newton is still the standard one in French.
The cogito has an ancestor outside Europe. Avicenna’s “floating man”, six centuries earlier, is created suspended in the air with no senses and still knows that he exists. Franklin, Tennyson, Aesop and Gandhi appear under this school on our cards; they belong to the wider temperament of reason and diligence rather than to the seventeenth-century system, which is how we use the word.
The school descends from Plato’s innate ideas and uses Aristotle’s logic while demolishing his physics. The Romantics are the direct reaction against it; the Existentialists are the later reaction against system-building of any kind. Its method of doubt is the Contrarian habit made rigorous.
Cartesian coordinates, from the man who watched a fly on the ceiling and wondered how to say where it was. Einstein, asked whether he believed in God, answered that he believed in Spinoza’s God. Every argument about whether the mind is the brain is Descartes’s argument, still open. And “the best of all possible worlds” is now only ever said the way Voltaire said it.