In order to seek firm foundations of sciences, Descartes operates hyperbolic doubt in Meditations which considers all doubtful things as false. And he introduces two hypothesis of "God who is all powerful(Deus qui potest omnia)" and demon for radicalizing the methodical doubt. The first problem concerning the divine deception is to know if and how human spirit can liberate himself from so called all powerful external cause. The aim of this article is to show that, contrary to most of scholastic theologians, Descartes didn't escape from the divine deception by appeal to the goodness of God(bonitas Dei). The second problem is to know whether human sprit, which is liberated from the divine deception, has possibility to cognize truths. In this paper, I will show that, contrary to scholastic theologians who thought that, because of the original sin, human being is not able to attain truths without supernatural and external help, Descartes admits to human spirit ability to cognize truths by himself. He also thought of a demonstration of God who doesn't lie as necessary condition for science. The knowledge of this God assures the fact that, freed from possibility of doubts which are grounded on the divine deception, human being is able to attain truths by himself.