In which discipline/field was René Descartes famous?

René Descartes (Latin "Renatus Cartesius"), born in La Haye en Touraine, now Descartes, Indre-et-Loire, France, on 31 March 1596 - 11 February 1650, was a French philosopher, mathematician, and scientist. He is the author, among other books, of "Discourse on Method".

"Cogito, ergo sum", (Latin: “I think, therefore I am") is a dictum coined by him in his “Discourse on Method” (1637) as a first step in demonstrating the attainability of certain knowledge. It is the only statement to survive the test of his methodic doubt.

In Cartesian philosophy, a way of searching for certainty by systematically though tentatively doubting everything. The doubt is methodic because it assures systematic completeness, but also because no claim is made that all—or even that any—statements in a dubitable class are really false or that one must or can distrust them in an ordinary sense. The method is to set aside as conceivably false all statements and types of knowledge that are not indubitably true. The hope is that, by eliminating all statements and types of knowledge the truth of which can be doubted in any way, one will find some indubitable certainties.

The statement is indubitable, as Descartes argued in the second of his six “Meditations on First Philosophy” (1641) because even if an all-powerful demon were to try to deceive him into thinking that he exists when he does not, he would have to exist in order for the demon to deceive him. Therefore, whenever he thinks, he exists.