I think, therefore I exist
This is not a quote from an ancient author, but from a seventeenth century French philosopher. Of course, Rene Descartes (1596-1650). Although Descartes chose to write his seminal work, the Discourse on Method, in French (so that, in his words, "even women can understand it") and not in the international scientific language of the time, Latin, the quote that condenses its fundamental idea has become famous in that latter language and not in its original version: "Je pense, donc je suis." The Discours de la methode was first published in 1637 as an introduction to a number of other studies. A Latin version was finally published in 1656 under the title Dissertatio de Methodo
Cogito ergo sum is one of the most popular Latin quotations. Its meaning is not, however, equally well known. Descartes's assertion may seem, at first glance, trivial, but it must be understood in its context. In the sixteenth and seventeenth centuries, philosophical skepticism was in a boom phase. It was a time of bloody wars of religion in which opposing dogmas generated in intellectual circles a strong tendency to relativism. To this was added the influence of the popularity of some ancient skeptic philosophers such as Sextus Empiricus. Descartes's goal in his Discourse on Method was to fight this skepticism and to provide a basis on which human knowledge could be developed. In the search of that unquestionably principle, Descartes applies systematic doubt to reject any assertion with the slightest possibility of falsehood. In this way he discovers that one can doubt about almost anything: the information our senses give us is not reliable, we easily fall prey to illusions, and there is no guarantee that the everyday world we experience is not just a fantasy of our own conscience or -as suggested by Descartes himself- the deception of a malignant deity.
Doubt seems, therefore, to destroy everything and leave no solid foundation for knowledge, but Descartes finds a point at which doubt must necessarily stop: there is a person who is thinking that everything is false and, if this person is thinking, this means she exists and that cannot be doubted. That is the idea expressed in the quotation under discussion, Cogito ergo sum. In his search for a method based on a totally rational foundation, Descartes finds a fundamental and undeniable truth: the existence of the self as the basis of all acts of unquestionable knowledge.
After finding this preliminary truth, Descartes extracts from it a general criterion that allows him to identify other truths: every simple intuition must be true or, put another way, all clear and distinct ideas must be true. I think that Descartes here contradicts the first and central of the four methodological principles he had laid down in the second part of his Discourse on Method: "not to accept as true anything, if there is not evidence that it is". Now, that an idea is clear and distinct is not enough evidence of veracity, besides, the clarity of an idea is appreciated differently by different people. As the humanist, historian and philosopher Giambattista Vico (1688-1744) said, if an idea seems us clear and distinct that does not mean it's true but only that we believe in it.