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What Did René Descartes Believe?

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  • Descartes (1596) invented the idea (named after him) of 'Cartesian Dualism', the idea that the mind and the body are made of two different substances and that the body is just something that occupies space in the universe whilst the mind is what does the thinking for us.
  • Descartes concerned himself with the nature of knowledge (epistemology) and spent his time pondering whether we really know anything for sure.
  • Descartes hated doubt and tried to eliminate it from his life through making a point of reasoning everything he thought he knew.
  • After thinking hard, he decided that there is almost nothing that we can be 100% sure of on the basis that our senses deceive us (e.g. oars look bent in water, optical and auditory illusions can trick us).
  • Descartes is most famous for his quote "cogito ergo sum" meaning 'I think therefore I am' stating that in order to question whether we really know anything, we must acknowledge that something is indeed posing that question and that therefore whatever it is that is thinking knows that itself exists. Descartes uses this as the foundation of his knowledge, stating that without realising the existence of god, our own existence is the only thing that we can be certain about.

Note: It is a common propogated myth that Descartes came up with the idea of "cogito ergo sum", in fact, the famous philosopher Aristotle noted exactly the same thing in his work 'Nicomachean Ethics'.

Descarte's 4 Rules for the Direction of Thought

These are rules he abided by when trying to decide whether something was true or not. In his own words:

  1. "Never to accept anything as true that I did not know to be evidently so... to include in my judgements nothing more than what presented itself so clearly and so distinctly to my mind that I might have no occasion to place it in doubt." Here he states that when making a more complicated decision, he would only use evidence that was self-evident (you cannot ever question that they are wrong e.g. triangles have three sides).
  2. "The second, to divide each of the difficulties that I was examining into as many parts as might be possible and necessary in order to solve it" meaning that he would split a decision into its fundamental elements. This is because that way he can more easily apply self-evident facts which he outlines in 1.
  3. "The third, to conduct my thoughts in an orderly way, beginning with the simplest objects and the easiest to know, in order to climb gradually" - similar to 1. he emphasises that he always builds up from small, clear facts to more complex truths.
  4. "...And the last, everywhere to make such complete enumerations and such general reviews that I would be sure to have omitted nothing." - here he states that he believed checking his reasoning over several times was very important in order to prevent the possibility of missing out an important element to the decision or even perhaps making mistakes in his reasoning.

Note: These may all seem like very simple and commonplace steps in reasoning to you, but at the time Descartes' method was heralded as very thorough.

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Descartes and God

  • Descartes was a believer of God.
  • He deduced that as well as the cogito, there must also be other things that we can be sure about: things that are intrinsically true. He concluded that shapes will always have their geometrical properties - triangles will always have 3 sides etc.
  • He reasoned that this can be applied to god and reasoned him into existence in the following way:

1. God is the perfect being.
2. Descartes is an imperfect being.
3. Descartes, being an imperfect being cannot possibly invent a perfect being.
4. Therefore, God must exist and have given Descartes the idea of God.

Descartes then states that God being perfect is omnibenevolent (all loving) and therefore God cannot be the cause of the deception to his senses that optical illusions and so forth bring to him. He concludes therefore that there is a powerful demon that does everything he can to lead humanity astray. This, he claims, is why our senses become confused.

  • Descartes therefore lived his life with the idea that he can trust his senses most of the time (since God does not deceive him) but sometimes the evil demon will deceive him and his senses will be wrong.


This is the real weakness of Descartes work because he brings God into existence through fallacious reasoning. One, how can Descartes be sure that he is not the same perfect being as he described God to be? If he already believes that he cannot know anything for sure based on his senses, then he cannot know that he is not the perfect being. Two, he claims that an imperfect being cannot invent the idea of a perfect being, but can we not imagine something without fully understanding how it works? We can imagine spaceships or hover boards without knowing how exactly they work or would work. Human imagination is not limited to what exists and therefore it is wrong to assume that just because we can't understand how something we imagined works, someone else must have put the idea in our heads. After all, who put the idea of a hover board in our heads and why? Simple questions like this break down Descartes' argument.

Ever seen one of these? Descartes invented it!

Ever seen one of these? Descartes invented it!

Descartes' Contribution to Mathematics

As well as his philosophical thought, Descartes is also famous for his huge contribution to mathematics. Some of his inventions include:

  • The cartesian co-ordinate system (the 2D co-ordinate system that uses X and Y axes in graphs)
  • Thus he is credited with the development of analytical geometry (applying algebra to geometry).
  • He also contributed to the understanding the laws of refraction and reflection.
  • He developed a rule for determining the number of positive and negative roots in an equation and considered that there could be negative roots as well as positive roots.
  • He created a symbol for powers, to show squaring and cubing: x2, x3 etc.

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