Assuming that medieval gnoseology is realist, this paper examines one of its features. Through a study of Thomas Aquinas, Summa Theologica, I, q. 84, a. 1 and Henry of Ghent, Suma, a. 1, q. 1, we try to show how certain gnoseological problems are solved by using the argument that assumes our cognitive faculties serve to know the reality as it is. We specially focus on Henry of Ghent, who uses it in order to invalidate the question of the possibility of knowledge.