Cambridge philosopher Iain King continues his series on the relationship between war and thought. Was the founder of modern philosophy a spy in the pay of a Catholic king?



René Descartes (1596-1650) was one of a world’s greatest-ever thinkers. His innovations in mathematics inspired calculus. His approach to science prepared the world for the Enlightenment. And his work in philosophy – his most significant achievement – finally deposed Aristotle, whose ideas had dominated Western thought for two millennia.

Cartesian co-ordinates were his second-most famous invention. Using just two or three axis, each set at right angles to each other, Descartes could define everywhere by a set of numbers. It was the revolutionary application of algebra to geometry, and it had clear military applications. Even unknown places were no longer boundless; Descartes was subjecting mystery to human control.

He pioneered optics, including theories on refraction and reflection. He calculated that rainbows form at a 42˚ angle from a sunbeam, and that they move with an observer. His genius was to suggest that some parts of the physical world did not depend on an observer (other than God, in which he was a firm believer), thereby setting the hard sciences free. Modern physics and chemistry owe much to Descartes’ worldview.

It is that worldview which earns Descartes a place amongst the greatest thinkers of all time. Descartes believed in two worlds: one physical, one mental. People, he said, operate in both – their body in the real world, their mind in the realm of ideas. He set out to prove each of these worlds existed.

The world of things and objects was the creation of a perfect God, who would not deceive, he argued. While the mental world could be proved through his most famous dictum: ‘I think, therefore I am’ (or, Cogito ergo sum, to use Descartes’ Latin).

The phrase is important because it dispels scepticism: the existence of thought means that something must exist. More than that, it allows Descartes to put questions to good use. No longer does the search for evidence threaten all that we cherish: Descartes offers a new approach based on men and ideas, rather than an unfathomable God.

Descartes’ ideas wrestled the modern era from an ancient one, and they earned him the title ‘Father of Modern Philosophy’.

How war shaped Descartes

The army of Gustavus Adolphus enters Frankfurt during the Thirty Years War. The conduct of military operations was increasingly scientific in character. It was also increasingly intelligence-driven. In both respects the young soldier René Descartes may have been influenced.

The army of Gustavus Adolphus enters Frankfurt during the Thirty Years War. The conduct of military operations was increasingly scientific in character. It was also increasingly intelligence-driven. In both respects the young soldier René Descartes may have been influenced.

Before he was a philosopher and mathematician, René Descartes spent three years as a soldier. But there are good reasons to think he secretly continued his military service for much longer – as a spy.

Aged 22, Descartes enlisted to fight in 1618, soon after Thirty Years War began with the Bohemian Revolt in Prague. Although he was a Catholic (he had been schooled by Jesuits), Descartes signed up with a Protestant Prince, Maurice of Nassau.

Initially based in Breda in the Netherlands, Descartes was trained – and then trained others – in military engineering. He would have studied the trajectories of cannonballs, and the use of building and mining to aid defence and attack. It is easy to imagine this time inspiring his thoughts about geometry.

Also, it was while stationed in Breda that he was mentored by one of the leading mathematicians of the day, Isaac Beckman. Descartes’ first year as a soldier was more about learning than fighting.

Either angered by squabbling within the so-called ‘United Provinces’ of the Netherlands, or at the direction of his spymaster, he soon switched allegiances to the Catholic Duke Maximillian of Bavaria. Military manoeuvres took him east, and while stationed somewhere near Neuburg, on the Danube, in November 1619, he had his ‘night of visions’. This was when he imagined the ground-breaking ideas at the heart of his philosophy, including his famous realisation ‘I think therefore I am’.

Descartes went on to witness the crucial Battle of the White Mountain of November 1620, which marked the end of the Bohemian Revolt. He abandoned soldiering soon after. By 1622 he had returned to Paris, and he spent most of the remaining 28 years of his life in the Netherlands.

There he wrote and published his ideas, before taking up a lucrative offer to become private tutor to Queen Christina of Sweden. It was a bad move: he died in February 1650, probably of pneumonia, brought on by the frigid Scandinavian air and too many early-morning teaching sessions, as demanded by the Queen (although some have suggested he was assassinated for trying to convert the Queen to Catholicism).

The case for Descartes being a spy is circumstantial, but the very nature of intelligence work, and the fluid alliances of the Thirty Years War, mean evidence is bound to be sparse. Descartes’ travels alone would raise eyebrows to any modern-day spy-hunter.

He was present at the coronation of the Holy Roman Emperor, Ferdinand II, in 1619, apparently having snuck into the cathedral uninvited. He visited courts and armies around Europe, concentrating on the most politically important places, while writing very little about what he was doing there (even though he wrote in so much detail about more trivial matters).

Most tellingly, he was a member of the Jesuits – one of the most committed and ideological of the Catholic orders – yet spent his time during a fierce religious war living in the heartland of Protestantism, where he enjoyed a comfortable lifestyle funded almost entirely by money from France (although he had a plausible cover story: that his income derived from a large family inheritance, invested in bonds).

Whether or not it is true, the hypothesis that Descartes was a spy provides a poetic explanation of his ideas. Here was a man committed to dispelling mysteries, who was a man of mystery himself. Setting out dualism – the notion that people have two selves, a body and a mind – Descartes is almost admitting his own double life, perhaps even sharing a secret joke with the select few who knew.

He could have used his mathematical skills to encode his messages; his system of coordinates allowed anyone to be pin-pointed – except Descartes himself, who remained elusive and regularly changed address, as would any modern spy.

Most important of all, Descartes-the-philosopher teaches us never to take anything for granted. His ideas are about doubt bringing its own benefits; about questioning what is before us, so we can be sure of what is left and build our knowledge base from solid foundations. It is exactly the lesson we would expect from a ‘retired’ soldier who worked for many years undercover, in the field of military intelligence.

The Thirty Years War

The Thirty Years War (1618-1648) was not a single confrontation, but a succession of conflicts; a war of several phases. Often overlooked from an Anglo-American perspective – partly because Britain’s role was peripheral – the Thirty Years War was probably more significant for the formation of modern Europe than, say, the Napoleonic Wars. Its impact persists to this day.

The war began in Prague, when representatives of the Catholic king-elect were thrown through a castle window: the Protestants of Bohemia were rebelling against the Holy Roman Empire.

After some inconclusive fighting, both sides brought in allies, and a much larger confrontation ensued – with the Catholic League, including Spain, Bavaria, and the Papacy, pitted against Protestants from the Netherlands and much of modern-day Germany, in alliance with the (Muslim) Ottoman Turks.

Spain and the Holy Roman Empire took almost three years to crush the revolt, by which time fighting had spread to France, the Netherlands, and along the Rhine. But Denmark feared a restored Catholic presence on its southern border, and intervened in 1625. Sweden – also Protestant, but at odds with Denmark – made even further inroads from 1630 onwards. Finally, France, which had initially been part of the Catholic League, switched sides and, allied with Sweden, won several victories, culminating in a final triumph at the Battle of Prague of 1648.

The war concluded where it had begun: in Prague Castle, which was taken by a Swedish flying column. But the victors failed to capture the eastern bank of the Danube, leaving Austria largely intact. The Holy Roman Empire, and the Habsburg dynasty of Spain and Austria, had been humbled – but would survive.

The Treaty of Westphalia, which formalised the peace, established the notion of modern nation-states – a sovereign territory with clear borders. Germany would remain a patchwork of such states for another two centuries, making France the rising force in European affairs.

Efforts to re-establish Catholicism throughout Europe had been banished, and never again would religion inspire such large-scale warfare in the Continent. The belligerents were too exhausted for that: the protracted fighting, widespread atrocities against civilians, along with plague and famine, had killed more than half the population through large swathes of Central Europe.

Leave a Reply