René Descartes – How to be certain

Some people claim that there is no truth. According to these sceptics everybody is stuck in their own personal perspective. Nobody can ever know anything for sure.


Then again dogmatists believe in only one set of beliefs, which must not be questioned at all. Everybody must believe the same things, whether they are true or not.


About 400 years ago René Descartes was confronted with a similar situation: On the one hand radical sceptics said that no knowledge was possible, and on the other hand dogmatists recognised only one source of truth: the church. The result was confusion and brutal conflict – over beliefs.

But Descartes wouldn’t have it. He wanted to know, not just believe. There had to be an alternative to nihilism and righteousness of faith! On a quest for clarity and certainty, he aimed to prove the radical sceptics wrong.

Here are five steps he took:

1. Think for yourself

He started with a thought experiment, in which he took the sceptic position to the extreme and doubted everything. Everything might be an illusion. He even doubted the existence of his own body.

He came to the conclusion that even if someone doubts everything, one thing remains absolutely certain: There must be something that is doing the doubting –  there must be something that is thinking.

i think therefore i am

At first sight his famous sentence “I think therefore I am” might seem banal, but it isn’t! This is our corner stone, the very thing that we can be absolutely sure of. It can not be doubted, not even by the most radical sceptics. It is the foundation on which we can build all our knowledge, said Descartes.

This also means that it is you, the thinking individual, not some big institution or tribe who is capable of coming up with true insights.

2. Be objective

Descartes separated this thinking instance from our bodies. After all, our bodies might still just be an illusion.

ghost in the machine

What is so interesting about this step is that the separation of mind and body makes an objective understanding of the world possible. For instance, we might disagree on whether a tree is small or big based on our personal subjective views. But we can agree on an objective statement that the tree is 2 meters high.

subjective vs. objective

We distance our thinking from our personal situation to take a neutral, an objective position. This objective “view from nowhere” is essential for any kind of natural science!

3. We might be wrong

But so far Descartes had not solved our main problem yet: Everything might still be an illusion – except for the thinker. Descartes’ way out of this problem was God. He claimed that there must be a benevolent God who has placed us in the world and given us the ability to understand just what we need, but not everything.

If – like me – you feel uncomfortable with this concession to dogmatic religious belief, then we may depart a little from Descartes and try out “nature” or “evolution” as the source of our abilities. In such a secular way of thinking we could say that evolution has given us just enough abilities to survive and procreate.

limited perspective

The point is: Descartes admitted that our cognitive abilities are limited. We are part of the creation, and we can understand parts of it. And yet we are prone to error.

4. Follow a solid method

But we can minimise that error, if we follow a rigorous method in our way of thinking. Descartes compared his own method with the rebuilding of a house. Sometimes one has to accept that a building is in such bad shape that it needs to be completely dismantled and rebuilt.


In a similar fashion, Descartes suggests that we should not dwell in the ruins of old beliefs. We should take them apart, and find reliable building blocks of simple truths that we can be absolutely sure of. Then we can carefully build our conclusions on this solid foundation. And “careful” means that we build rational, logically sound arguments.

good arguments

To think scientifically does not mean to simply cite facts that come from reliable authorities – whether that is other scientific studies, institutions, friends or famous people. It is a method of thinking, a method of deducting complex truths from first principles yourself.

5. Do the Math (I know… but stay with me!)

As an ideal example for such a method Descartes admired mathematics. We can start with simple observations that we find intuitively true: for instance, that if we add three apples to two apples we have five apples.


From this observation we can derive basic principles of how to add numbers. They are both intuitive and evident. Then we can carefully build on these simple principles and conclude that the additions of larger numbers must also be true. We know that they are true, even though we don’t – or in extreme cases even can’t! – verify them by observation.

Descartes also promoted the combination of geometry and algebra. One consequence is the coordinate system later named after Descartes. This is also the basis for the very screen you are looking at! The position of every pixel on your screen can be represented with two numbers: x and y.

cartesian coordinates

We can add a third dimension z and thus represent every object in space with numbers. This makes it easier to create mathematical models of the world. Thus mathematics is a reliable connection between us and the world!

cartesian coordinates

Today we are used to math as an elementary tool of the natural sciences but it was people like René Descartes who laid the foundation.

How this can help us

There is plenty of criticism to be held against Descartes, starting with his shaky proof of God, the alienation from our bodies to the conviction that only what can be expressed in numbers counts. And Descartes would probably encourage you to critically examine his ideas yourself.


But regardless of whether this criticism is fair or not – let us remember that his method of individual rational thinking has been a way out of the predicament of scepticism and dogmatic belief. It prevents much unnecessary conflict and confusion. It is foundational ideas of the enlightenment like Descartes’ that make it possible for us to communicate and interact constructively and improve our understanding of the world. Descartes’ words are as relevant today as they were 400 years ago.

If you want to dig deeper

To get started with Descartes’ original texts I highly recommend his Discourse on Method. It is rich with fascinating thoughts and advice.

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