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. Where there is a being in change, i.e., passing from potency to actuality, there must be another being actually existent, outside the series in change, whether this series is considered to be finite or infinite.
Aquinas formulates this principle in three different ways according to the three aspects of reality taken into consideration. For the first way the formulation is: What is moved, is moved by another; for the second way: It is impossible for something to be the efficient cause of itself; for the third way: What is not, cannot begin to be, unless by force of something which is.
The fourth way takes into consideration many aspects of reality, which, when compared with one another, show that they are more or less perfect. The principle of intelligibility is the following: What is said to be the greatest in any order of perfection is also the cause of all that exists in that order.
The fifth way takes into consideration the order of nature: Where there is a tendency of many to the same end, there must be an intellectual being causing such an order.
Let us set forth the schematic structure of the five ways:
(1) Our senses attest to the existence of movement or motion. But every motion presupposes a mover which produces that movement. To have recourse to an infinite series of motions is not possible, for such an infinite series does not and cannot solve the question of the origin of the movement. Hence there exists a first mover that moves and is not itself moved. This is God.
(2) Some new thing is produced. But every new production includes the concept of cause. Thus there exists a first cause which is itself not caused. This is God.
(3) Everything in the world is contingent; that is, it may or may not exist. We know from experience that all things change in one way or another. But that which is contingent does not have the reason of its existence in itself, but in another, that is, in something which is not contingent. Hence there exists the necessary being, God.
(4) The fourth way takes into consideration the transcendental qualities of reality, "the good, the true, the noble," and so forth, which we find in things to a greater or lesser degree. But transcendental qualities are nothing other than being, expressed through one of its attributes; hence things under our experience are beings to a greater or lesser degree. But the greater and lesser are not intelligible unless they are related to that which is the highest in that order; and what is the highest is also the cause of all that exists in that order. Therefore there exists the highest degree of being and it is the cause of all limited being. This is God.
(5) Order exists in the world about us. Hence there must exist an intelligence responsible for the order of the universe. This is God.
Thus, in brief, we have Aquinas' five proofs for the existence of God; proofs from the notion of motion, cause, contingency, perfection, and order.
The proofs for the existence of God are also means of knowing something of God's essence. This knowledge, however, remains always essentially inadequate and incomplete.
One way of knowing God is the way of negative theology, that is, by removing from the concept of God all that implies imperfection, potentiality, materiality. In other words, by this method we arrive at a knowledge of God through considering what He is not.
A second method is that of analogy. God is the cause of the world. Now every object reflects some perfection of the cause from which it proceeds. Hence it is possible for the human mind to rise to the perfections of God from the consideration of the perfection it finds in creatures. This it does, naturally, by removing all imperfection and potentiality from the creatures considered. The resultant idea of the nature of God is thus had through analogy with the perfections of the created universe.