In his critique of Aristotelian epistemology, Peter John Olivi (1248-1298) shows how the soul can know itself. Against the position of Aristotle, the intellect can only produce a fictional image. A second possibility of an immediate reflexive knowledge is therefore necessary in order for the subject to reach certainty. This inner intuition takes place through a return of the mens on itself : the movement of conscience, as Olivi defines it. That being said, it is in the notion of personality that the subject’s evidence is truly realized. After Boethius and Richard of Saint Victor, Olivi elaborates a new definition of person, as an existence through oneself, free, and capable of an immediate return to itself. In so doing, Olivi not only introduces the notion of conscience in the definition of the person, he also sees the subject as the source and principle of any knowledge.