Descartes

René Descartes (1596-1650) has the merit of making us approach total doubt with confidence, optimism, love for research, rather than wishing to abolish it because of a preconceived fear of anarchy, of the total disorientation to which it can lead. He realized that there is no method for distinguishing between dreaming and reality; we can even be suspicious about our belief that two plus two makes four; however, in his reflection path, doubt leads to an indubitable certainty: “cogito, ergo sum”, that is “I think, therefore I am”. Descartes knew that this reasoning had already been done about 1200 years before by St. Augustine (354-430 AD), but the latter had not assumed it as the basis of a whole philosophy.
Once we appreciate the merit of Descartes, we must not hide, however, that his conclusion is actually further criticizable. It may be useful to realize some questions to which it does not give an answer: who said that, in order to start a knowledge, we must necessarily start from a fixed point? Why should we consider total doubting as something to overcome, an enemy to fight? Who authorizes us to pass from the datum of doubting to the datum of existence? Who guarantees us that the evidence of our existence is not a deception itself? What does it mean to exist? What does it mean to doubt?
Another important aspect of Descartes is mechanism: the material world (distinct from the supernatural one, of which God and the soul are a part, for example) is nothing but a gigantic machine, like a big clock created by God, and therefore explainable in all its phenomena through the geometric laws of matter, which occupies a space, and of movement. However, Descartes does not deny the existence of free will.
We should not ignore that even mechanism is open to criticism: geometric ideas are human ideas, adaptations of phenomena to the comprehension abilities of our brain; therefore we must not forget that it is not the world that obeys the laws that we have discovered, but it is those laws, these little tools for understanding, created by our brain for its utility, that have a duty to continuously listen to the world, from which they were obtained.
The Cartesian rules of provisional morality are extraordinarily modern: 1) in practical life it is inevitable having to act being content with truths that are not entirely evident, otherwise we would remain eternally undecided; 2) once a decision has been made, we must be resolute and not doubting endlessly; 3) rather than wanting to change the order of the world, it is better to try to improve our thoughts first.