The search for God and His relationship with the world was as fundamental in the Middle Ages as it was at any time during the history of Christian thought. At the time of Aquinas, Augustinianism was the most appreciated doctrine in the school of philosophy at the University of Paris.In virtue of illumination, which is the central point of Augustinianism, the human soul could have an intuitive knowledge of God. Indeed the intellect had only to reflect upon itself to find the presence of the Divine Teacher.Thus the existence of God was proved a priori by means of necessary reason. Obviously, if the presence of the ideas of absolute truth and good in our mind must be explained by the direct suggestion of God, we do not need any other proof of God's existence.But, according to Aquinas, any natural intuitive knowledge of God is precluded to man. For us, only the visible world, which is capable of impressing our senses, is the object of natural intuitive knowledge. Thus any argument a priori for the existence of God is devoid of validity.For Aquinas, the existence of God needs to be demonstrated, and demonstration must start from the sensible world without any prejudice. (1) Such demonstrations are possible and are accommodated to anyone who is simply capable of reflecting.There are five ways in which the human intellect can prove the existence of God. All have a common point of resemblance. The starting point is a consideration of the sensible world known by immediate experience. Such a consideration of the sensible world would remain incomprehensible unless it was related to God as author of the world.So each argument might be reduced to a syllogism whose major premise is a fact of experience, and whose minor premise is a principle of reason, which brings to light the intelligibility of the major premise.It is interesting to note that Aquinas uses the Aristotelian principle of the priority of act over potency for the first three arguments...
