Summa contra Gentiles1.12 Chapter XII. Of the opinion of those who say that the existence of God cannot be proved, and that it is held by faith alone

The position that we have taken is also assailed by the opinion of certain others, whereby the efforts of those who endeavour to prove that there is a God would again be rendered futile. For they say that it is impossible by means of the reason to discover that God exists, and that this knowledge is acquired solely by means of faith and revelation.

In making this assertion some were moved by the weakness of the arguments which certain people employed to prove the existence of God.

Possibly, however, this error might falsely seek support from the statements of certain philosophers, who show that in God essence and existence are the same, namely that which answers to the question, What is He? and that which answers to the question, Is He? Now it is impossible by the process of reason to acquire the knowledge of what God is. Wherefore seemingly neither is it possible to prove by reason whether God is.

Again. If, as required by the system of the Philosopher, in order to prove whether a thing is we must take as principle the signification of its name, and since according to the Philosopher (4 Metaph.) the signification of a name is its definition: there will remain no means of proving the existence of God, seeing that we lack knowledge of the divine essence or quiddity.

Again. If the principles of demonstration become known to us originally through the senses, as is proved in the Posterior Analytics, those things which transcend all sense and sensible objects are seemingly indemonstrable. Now such is the existence of God. Therefore it cannot be ' demonstrated.

The falseness of this opinion is shown to us first by the art of demonstration, which teaches us to conclude causes from effects. Secondly, by the order itself of sciences: for if no substance above sensible substance can be an object of science, there will be no science above Physics, as stated in 4 Metaph.' Thirdly, by the efforts of the philosophers who have endeavoured to prove the existence of God. Fourthly, by the apostolic truth which asserts (Rom. i. 20) that the invisible things of God are clearly seen, being understood by the things that are made.

Nor should we be moved by the consideration that in God essence and existence are the same, as the first argument contended. For this is to be understood of the existence by which God subsists in Himself, of which we are ignorant as to what kind of a thing it is, even as we are ignorant of His essence. But it is not to be understood of that existence which is signified by the composition of the mind. For in this way it is possible to prove the existence of God, when our mind is led by demonstrative arguments to form a proposition stating that God is.

Moreover. In those arguments whereby we prove the existence of God, it is not necessary that the divine essence or quiddity be employed as the middle term, as the second argument supposed: but instead of the quiddity we take His effects as middle term, as is the case in a posteriori reasoning: and from these effects we take the signification of this word God. For all the divine names are taken either from the remoteness of God's effects from Himself, or from some relationship between God and His effects.

It is also evident from the fact that, although God transcends all sensibles and senses, His effects from which we take the proof that God exists, are sensible objects. Hence our knowledge, even of things which transcend the senses, originates from the senses.