Descartes and Voltaire: Rationalism and Enlightenment

René Descartes (1596–1650) was a French mathematician, scientist and philosopher. He wanted knowledge that nobody could shake. He did not want opinions that only seemed likely.

What was he looking for?

He looked for a firm foundation. Think of a building. A strong building needs solid ground and then walls built on it. Descartes wanted knowledge built the same way. First a certain foundation, then everything else on top.

Why did he not trust what he had learned?

He studied at one of the best schools in Europe. Yet he found himself full of doubts and errors. He felt he had only discovered his own ignorance. So he decided to test his beliefs himself.

Descartes used a simple rule. If a belief had even a small reason for doubt, he set it aside as if it were false. He then checked whether anything was left that nobody could doubt.

What did he doubt first?

He doubted the senses. Our senses sometimes deceive us. So he would not rest his knowledge on them alone.

What is the dream argument?

Descartes noticed that dreams feel real while we dream. So how can we be sure we are awake right now? He supposed that all he had ever seen when awake had no more truth than a dream.

What is the evil genius?

This is the strongest doubt. Descartes imagined a powerful and cunning deceiver. This deceiver uses all his energy to mislead him. The doubt is a thought experiment. Descartes did not believe the deceiver was real. He used it to test his beliefs to the limit.

Even while he doubted everything, one fact stayed. He was doubting. And doubting is a kind of thinking.

What does “I think, therefore I am” mean?

In Latin it is “cogito ergo sum”. Descartes wrote that even if he thought everything false, he who thought must be something. The thinker cannot doubt that he exists while he is thinking.

Why was it so important?

Even the sceptics could not shake it, he said. So he accepted it as the first principle of his philosophy. It became the foundation he had been looking for. The aim of the doubt was certain knowledge, not permanent doubt.

Where did he write this?

The line appears in the Discourse on the Method, published in French in 1637. The Meditations on First Philosophy followed in Latin in 1641.

Voltaire is the pen name of François-Marie d’Arouet (1694–1778). He was a French writer and public activist. He played a singular role in shaping the Enlightenment.

How did England change him?

He lived in England in exile from 1726 to 1729. The trigger was an affair of honour with a powerful aristocrat, the duc de Rohan. His English years shaped his thought deeply.

What tools did he use?

Voltaire did not write only heavy philosophy. He used fictional stories, satires, poems and pamphlets. Short and sharp writing could reach many readers. That made criticism public.

Where did he spend his last years?

He settled at Ferney, near the Swiss border, in early 1759. He lived there until his death in 1778. From Ferney he kept up his campaigns.

Candide was first published in 1759. It follows a wandering traveller through farcical adventures. Behind the jokes is a serious attack on the philosophy of Leibniz.

What is the idea being mocked?

That idea is optimism: the claim that this is the best of all possible worlds. In the story, the tutor Pangloss repeats it, even as terrible things happen to everyone.

How does the story end?

Candide gives a plain answer: “we must cultivate our garden”. It means stop empty theory and do useful work.

Why does the book matter?

Voltaire made a heavy argument easy to remember. A reader laughs first and thinks afterwards. That is the power of satire.

Jean Calas was a French Protestant. Voltaire argued he was a victim of a despotic state and a cruel judicial system. Calas had been persecuted by a Catholic monarchy.

What did Voltaire do?

He mounted a public defence. He wrote A Treatise on Tolerance in 1762.

Did Voltaire say “I disapprove of what you say”?

Probably not. The famous line is often credited to Voltaire. But the earliest evidence is the 1906 book The Friends of Voltaire by S. G. Tallentyre. That was the pen name of Evelyn Beatrice Hall. Her words described what she took to be his view.

What did they share?

Both asked people to test inherited authority with their own reason. The Enlightenment is described as a drive for intellectual progress, with philosophy standing as an independent force that can challenge the old.

How did they differ?

Descartes worked alone on questions of certainty. Voltaire took on public injustice. Voltaire also criticised philosophers such as Descartes and Leibniz.

Why does this matter for exams?

Questions often ask for one shared goal. The answer is to subject inherited authority to critical, reasoned scrutiny.

• “I think, therefore I am” is Descartes. “The unexamined life is not worth living” is Socrates.
• Candide is Voltaire’s novel. Meditations is Descartes. Leviathan is Hobbes. The Prince is Machiavelli.
• The method of doubt aims at certain knowledge. It does not aim at permanent doubt.
• Voltaire used satire, not force or silence.
• The line “I disapprove of what you say” is not found in Voltaire’s own writing.

What is the memory hook?

Descartes doubts to find certainty. Voltaire laughs to find freedom.