Descartes' Discourse on the Method (1637): Cogito, Ergo Sum

The Discourse on the Method, published in Leiden in 1637, is the short French book in which René Descartes set out a new way of seeking truth by doubting everything that could be doubted. It contains the line “I think, therefore I am”, in Latin cogito, ergo sum, and it introduced three essays on optics, weather and geometry. It is often treated as the starting point of modern Western philosophy.

Background: a soldier-scholar in search of certainty

René Descartes was born on 31 March 1596 at La Haye in Touraine, France, a town now named Descartes after him. He was educated at the Jesuit college of La Flèche and took a law degree at Poitiers in 1616. He later wrote that his schooling had left him with many doubts and few certainties, apart from mathematics.

In 1618 he joined the army of Maurice of Nassau in the Netherlands, and later travelled in German lands at the start of the Thirty Years’ War. On the night of 10 November 1619, shut in a stove-heated room in southern Germany, he had a series of dreams that he took as a sign that he should rebuild all knowledge on a single sure method.

From 1628 Descartes lived mainly in the Dutch Republic, where he found quiet and relative freedom. By 1633 he had finished a treatise on physics, The World, which accepted that the Earth moves. When he heard of the trial of Galileo that year, he withdrew it from publication. The Discourse was in part his cautious way of presenting some of his results without the full system.

The book and its essays

The Discourse appeared anonymously in 1637, printed by Jan Maire in Leiden, with the printing completed in June. Its full title translates as Discourse on the Method of Rightly Conducting One’s Reason and Seeking Truth in the Sciences. Descartes wrote it in French rather than Latin so that people of good sense, not only scholars, could read it.

The Discourse itself is fairly short, divided into six parts and written partly as an intellectual autobiography. It was followed by three essays meant to show the method at work: La Dioptrique on light and lenses, Les Météores on weather and the rainbow, and La Géométrie on mathematics. Together they filled a volume of roughly 500 pages.

Descartes’ friend Marin Mersenne, a Minim friar in Paris who corresponded with scholars across Europe, helped arrange the French royal privilege and circulated the book among learned readers.

The method of doubt

In Part Two Descartes gives four rules. He will accept nothing as true unless it is so clear and distinct that he has no reason to doubt it; divide every difficulty into as many parts as possible; think in an orderly way, from the simplest matters to the most complex; and make reviews so complete that nothing is omitted.

Part Three offers a “provisional morality” to live by while everything else is in doubt. He would obey the laws and customs of his country and keep his Catholic faith, act firmly once he had decided, and try to master his own desires rather than fortune.

“I think, therefore I am”

Part Four is the philosophical heart of the book. Descartes resolves to treat as false anything open to the slightest doubt: the senses sometimes deceive, dreams can seem real, and even reasoning can go wrong. Yet while he doubts everything, he notices, he must exist in order to doubt. Hence “je pense, donc je suis”, “I think, therefore I am”.

From this first certainty he argues that he is essentially a thinking thing, distinct from the body, and that a perfect God exists who would not deceive him about what he perceives clearly and distinctly. This sharp division between mind and matter, later called Cartesian dualism, shaped centuries of debate. Descartes developed these arguments more fully in his Meditations on First Philosophy (1641).

Part Five summarises his suppressed physics. He describes the circulation of the blood, recently published by William Harvey, though he explained the heart’s motion by heat rather than muscle. He also argued that animals are complex machines without minds, a view that proved highly controversial. Part Six explains why he had held back his larger work and calls for experiments carried out by many hands.

The mathematics: La Géométrie

The most lasting scientific achievement in the 1637 volume was La Géométrie. Descartes showed how geometric curves could be described by algebraic equations and how equations could be pictured as curves. This union of algebra and geometry is called analytic geometry; the coordinate system named “Cartesian” in his honour grew out of it, though Descartes did not use perpendicular axes in the modern way.

He also introduced notation that is still standard, using letters at the start of the alphabet for known quantities and x, y and z for unknowns, and writing powers as raised numbers, such as x³. Pierre de Fermat had reached similar ideas independently around the same time.

La Dioptrique stated the law of refraction in the form now known as the sine law, which the Dutch scientist Willebrord Snell had found earlier but not published. Les Météores gave a largely correct account of the angles at which the primary and secondary rainbows appear.

Reception and consequences

The book drew objections at once, including from Fermat on optics and from scholars defending Aristotle. In the Dutch Republic, Cartesian ideas split the universities of Utrecht and Leiden, and Utrecht’s senate formally condemned the new philosophy in 1642.

A Latin translation, overseen by Descartes, appeared in Amsterdam in 1644, opening the work to readers across Europe. In 1649 Descartes accepted an invitation from Queen Christina of Sweden. He died in Stockholm on 11 February 1650, probably of pneumonia. In 1663 the Catholic Church placed his works on the Index of Prohibited Books.

Despite this, Cartesian philosophy spread widely. Thinkers such as Thomas Hobbes, Antoine Arnauld, Baruch Spinoza and Gottfried Wilhelm Leibniz engaged with it, and Isaac Newton learned his geometry partly from Descartes before reacting against his physics in the Principia.

Myths and facts

Myth: “Cogito, ergo sum” first appeared in the Discourse. The Discourse was written in French. The Latin phrase became standard through Descartes’ Principles of Philosophy and the Latin edition of the Discourse, both of 1644.

Myth: Descartes invented the x and y axes as drawn today. His geometry laid the foundations, but the familiar pair of perpendicular axes was developed by later mathematicians.

Myth: Descartes doubted for the sake of doubting. His doubt was a tool meant to reach certainty, not a sceptical end in itself.

Timeline

• 1596, 31 March: René Descartes is born at La Haye, Touraine.
• 1619, 10 November: Descartes has the dreams that set him on his search for a method.
• 1628: He settles in the Dutch Republic.
• 1633: After Galileo’s condemnation he withholds his treatise The World.
• 1637, June: The Discourse and its three essays are printed in Leiden.
• 1641: Meditations on First Philosophy is published.
• 1644: The Latin translation of the Discourse and the Principles of Philosophy appear.
• 1650, 11 February: Descartes dies in Stockholm.
• 1663: His works are placed on the Index of Prohibited Books.

Why it matters

The Discourse argued that each person can reach truth through reason and a careful method instead of relying on authority. Its search for a certain foundation for knowledge set the agenda for modern philosophy, and its essay on geometry gave mathematics and physics one of their most important tools.