Descartes, by A C Grayling <br/> Voltaire Almighty: A life in pursuit of freedom, by Roger Pearson

When God began to die

Karl Marx argued that "philosophers have always tried to interpret the world; the point is to change it". If only it were possible to do one without the other. In the 17th century, René Descartes swept away the teachings of Aristotle that had sustained philosophy and Christianity for centuries - but only, he hoped, in order to put the church on a firmer footing.

The new foundations he created were indeed firmer for something, but not for what he intended. Philosophers divide into "solvers" and "spoilers", those who make the world intelligible, and those who show us that we understand far less than we think. Those who would occupy the former camp have a tendency to find themselves rather in the latter. Thinkers who realise this in their lifetimes can acquire the unimpeachable moral status of a Bertrand Russell, an Albert Camus - or a Voltaire.

These new biographies by A C Grayling and Roger Pearson show how Descartes and Voltaire helped to make the modern world by undermining their religion. Voltaire finally ran out of patience with the assumed beneficence of God after the Lisbon earthquake of 1755, but he already found the Bible ludicrous and the authority of the church arbitrary and cruel. Descartes's contribution was unintended. He was shocked into radical thoughts not by natural disasters or the injustices meted out by his co-religionists, but by his own genius.

Intellectually, Descartes (1596-1650) did not know who his friends were. He hoped to put the core beliefs of Christianity on an indubitable footing in the face of the revolution begun by Galileo - a man he had far more in common with than any churchman. But his failure would do far more lasting damage than even Voltaire's direct attack.

Descartes's Jesuit education mirrored his later development, with its kindnesses and freedoms. It is unfortunate that the order's enlightened teaching methods were employed ultimately to instil a profound ignorance and intolerance in its charges. With equally good intentions, Descartes's hard-headed "method of doubt" ended up endorsing the same dusty superstitions shared by his benighted contemporaries.

For example, his orthodox belief in how souls were distributed allowed him to cut open a living dog's heart for an experiment while denying that the animal felt any pain. The philosopher had resolved to set aside all his beliefs until he found those upon which an edifice of knowledge could be reconstructed.

Famously, he settled on the fact of his own thinking, his own existence. But to move from this scrap of knowledge to a full understanding of the world, he employed the notion of a benevolent God who would furnish us with our most important beliefs. Certain ideas are sufficiently "clear and distinct" in the light of reason to count as knowledge because they come from God, who would not deceive us about them.

The project ended in embarrassment, because the existence of this God was itself a truth dependent upon the reliability of Descartes's reason. He carried on regardless, and dreamed that his teachings would find their way into school textbooks.

However, the authorities had a better understanding of how dangerous it was to tinker with their brittle dogmas. With his denial that God was required to micro-manage the universe day-to-day, Descartes initiated the Almighty's retreat into a "god of the gaps" - a retreat that, to his biographer's delight, has continued at speed ever since. In his heart a conservative, Descartes's fate was to be a revolutionary: philosophy's Mikhail Gorbachev.

Grayling writes of Descartes's method as a shining example to philosophers, even if the philosopher did not follow his own rules as assiduously as he might have. The same could be said of most great thinkers, their theories lying in the graveyard of ideas while their methods, tools and principles live on to inform further generations of philosophers. History appears to show that philosophy is equivalent to technology rather than a body of truths, and its course a development of ever-finer instruments rather than a long march towards an eternal Truth.

Less evident from this account is how Descartes has been the whipping-boy of philosophy for decades. So many theses today take care to begin with a denunciation of the first modern philosopher. Intellectually paranoid and unforgiving of imagined slights, France's greatest philosopher would have been horrified by his destiny.

Pearson's Voltaire had much more fun than poor Descartes, who might only ever have had sex a single time. Born almost a hundred years after his countryman, in 1694, Voltaire enjoyed his early life in Paris, his various romantic partners and his instant success as a playwright. Yet he wrote despite his country - just as all Descartes's published works were written after France's greatest philosopher left France.

In England, Voltaire was surprised to see Isaac Newton's body interred in Westminster Abbey alongside monarchs, and contrasted this with his own treatment in his Catholic homeland - which had included two stints in the Bastille. He also saw his former lover, Adrienne Lecouvreur, cast into a pauper's grave even though she had left her parish church a handsome sum of money. As an actress, she was automatically excommunicate, whereas in England her equivalent, Anne Oldfield, received the same treatment as Newton.

Voltaire found that he rose to the top only as an entertainer in the eyes of the aristocracy. His upper-class sponsors would close ranks when he entered into dispute with one of their own. The writer of Candide took his revenge in satire, but his heroism in the face of the censors is almost lost in the tide of minutiae in this account - from his lodgings and acquaintances right down to his daily meals.

It could have been so different. After all, as the author mentions, this is the man who kept a straight face when teasing Alexander Pope's mother that her son's ill health was owed to frequent buggerings as a boy.

Descartes had a colder life than Voltaire, but it is easier to warm to him here. Though less thorough, Grayling's is by far the more readable biography. This is probably because he is not overawed by his subject. Grayling gives us a pompous Descartes turning up in Paris to accept his royal pension, buying a green silk suit and renting the palatial apartment that would befit his new station, only to be sent scurrying away empty-handed by the popular uprisings in the capital.

When Queen Christina of Sweden employed him, against his protests, to write a comic libretto celebrating the end of the Thirty Years War, the philosopher protested that the idea was tasteless and hatched a scheme worthy of Max Bialystock in The Producers. However, his attempt to sabotage the piece by filling it with maimed soldiers and desperate refugees backfired. The Swedish public found them hilarious and demanded that he write a sequel.

It seems obvious today that the best way to undermine authority is to render it ridiculous. In this, Voltaire was the original and still the best. But when the laughing stops and the authority is still there, as the French found in 1789, the real work of philosophy then begins.

Nicholas Fearn's book 'Philosophy: the latest answers to the oldest questions' is published by Atlantic