The main purpose of the present text is to indroduce in three steps a philosophical thematization of a notion "cogito": First in the sphere of René Decartes, where it serves as a starting point, methodology or first philosophy, secondly in the perspective of Edmund Husserl, for whom it serves as a possible answer to his search for an apodictic evidence, and lastly in thought of Jean-Paul Sartre, who, in the given context, understands it as the deepest ground of subjectivity, in other words of existence itself.