Rene Descartes was a great French philosopher and mathematician during the 17th century. He is considered as a precursor to the Rationalist school of thought, and due to his vast contributions to the fields of philosophy and mathematics, he is often known as the ‘Father of Modern Philosophy.’

The Latin phrase “cogito, ergo sum” is a translation of Descartes’ original French statement, “Je pense, donc, je suis.” The argument that is usually summarized as “cogito, ergo sum” appears first in Descartes’ Meditations on First Philosophy, where he attempts to build an entire philosophical system with no prior assumptions. He reasons that since all his beliefs have been derived from potentially misleading sense data or potentially fallacious logic, he can trust nothing that he has hitherto taken to be true. That is to say, he decides to systematically doubt all that could conceivably be doubted. He discovers the one thing that he cannot doubt is his own existence. After all, he claims, something nonexistent is incapable even of the act of doubting. Thus the formulation, “I think, therefore I am”, was the starting point of his philosophy.

There are three important notes to keep in mind here. First, he claims only the certainty of his own existence from the first­person point of view — he has not proved the existence of other minds at this point. This is something that has to be thought through by each of us for ourselves, as we follow the course of the meditations. Second, he does not say that his existence is necessary; he says that if he thinks, then necessarily he exists (see the instantiation principle). Third, this proposition “I am, I exist” is held true not based on a deduction or on empirical induction but on the clarity and self­-evidence of the proposition.

The proposition is sometimes given as “dubito, ergo cogito, ergo sum.” This fuller form was penned by the eloquent French literary critic, Antoine Léonard Thomas, in an award-winning 1765 essay in praise of Descartes, where it appeared as “Puisque je doute, je pense; puisque je pense, j’existe.” In English, this is “Since I doubt, I think; since I think I exist”; with rearrangement and compaction, “I doubt, therefore I think, therefore I am”, or in Latin, “dubito, ergo cogito, ergo sum”.

According to many Descartes specialists, including Étienne Gilson, the goal of Descartes in establishing this first truth is to demonstrate the capacity of his criterion — the immediate clarity and distinctiveness of self­evident propositions — to establish true and justified propositions despite having adopted a method of generalized doubt. As a consequence of this demonstration, Descartes considers science and mathematics to be justified to the extent that their proposals are established on a similarly immediate clarity, distinctiveness, and self­evidence that presents itself to the mind.