The Evil-Demon-Argument

Transitioning argument from the senses, and imagination to understanding:

After having demonstrated that we can neither trust the senses, nor our images as representations of real objects, Descartes tries to find another ground for sciences, namely understanding (rationality). So even though we might be trapped in a dream, dreams are composed of elements and these elements are simple. So we can, for example, invent, different objects that are combinations of simple elements, but can we also invent elements?

For example, objects have colors, but can you invent a new color in a dream? Can you invent new shapes?

The following picture resembles an absurd labyrinth, but despite its absurdity it uses elements, in order to be represented in our mind.



So Descartes concludes that the simple elements might qualify as foundations for sciences, but he can also find doubt about this.

The Evil-Demon

So we can believe in the simple elements of understanding. These elements include arithmetic sentences like 2+2=4, geometrical shapes, or even the method of deduction. These statements are evident, but this is not necessarily an indicator that they are true.

The simple task of teaching a child of let us say 4 years the sentence 2+2=4 is much more difficult then we expect. Usually children can count until 4, but they cannot understand what it means to build a some. Numbers are not simple truths, but actually presuppose a complex set of different skills. Because we have trained this sentence for thousands of times, since our early school days, the sentence occurs as evident to us. But could it be that the school has deceived us into believing such simple sentences, so that we believe they are without really knowing this?

The following picture represents this problem. 

So the question of Descartes is legitimate. It could be that an evil teacher, or an evil demon has put these truths in our mind. 

Since Descartes finds the slightest doubt, he dismisses the belief in understanding as a whole.

Further Sources:

• Stories of different demons and paradoxes https://en.wikipedia.org/wiki/Demon_(thought_experiment)
• It is always good to be informed on Realism, a central concept of our time http://plato.stanford.edu/entries/realism/#2
• The wikipedia page gives background information on the evil demon https://en.wikipedia.org/wiki/Evil_demon