Disputed Questions on Truth, Question 10, Article 12:
Whether God’s existence is known through itself (per se notum) by the human mind just as the first principles of demonstration which cannot be thought not to be.
I respond that it should be said that there are found three opinions about this question.
For some have said, as Rabbi Moses relates, that God’s existence (Deum esse) is not known through itself, nor is it known from a demonstration, but is accepted only on faith; and the weakness of reasons which many put forward for demonstrating that God existence led them to this position. Others, such as Avicenna, said that God’s existence is not known through itself but is known through demonstration. But others, such as Anselm, thought that God’s existence is known through itself, insofar as, no one can understand interiorly (cogitare interius) that God does not exist, even though one can say this exteriorly, and understand interiorly the words by which it is said.
The first opinion appears manifestly false. For one finds the fact that the existence of God (is) proved by irrefutable arguments (rationibus irrefragabilibus) even by philosophers. Nevertheless, frivolous arguments also are put forward by some people for showing this.
Each of the other two arguments which follow are true in a certain respect. For something can be known through itself in two ways: namely in relation to itself, and in relation to us. Therefore, that God exists is known through itself in relation to itself, but not in relation to us; and so, in order for this to be understood, it is necessary for us to have demonstrations taken from effects. And this is clearly so.
In order for something to be known through itself, nothing else is required than that the predicate belong to the nature of the subject, for then the subject cannot be thought without that which is predicated being manifest in it. However, in order for it to be known through itself, it is necessary that the nature (ratio) of the subject, in which is included the predicate, be known by us. And so it is that certain things are known through themselves by everyone, namely when propositions have subjects of which the nature (ratio) is known by everyone, such as (that) every whole is greater than its part, for everyone knows what a whole is and what a part is. Certain (other) things are known through themselves by the wise alone who know the natures of the terms, common people being ignorant of them.
And according to this, Boethius says in the book De Hebdomadibus that there are two kinds of common conceptions. One is common to everyone, such as “if equal amounts are subtracted from equal things, what remain are equal to each other.” The other is (common) to the learned alone, such as “incorporeal things are not in a place,” which common people do not approve, but the learned do, since common people cannot transcend the consideration of imagination which pertains to the nature of an incorporeal thing.
However, being (esse) is included perfectly in the nature of no creature, for the being of every single creature is other than its quiddity. Wherefore, it cannot be said of any creature that its being is known through itself and according to itself. But in God, his being is included in the nature of his quiddity because in God what He is, and that He is, is the same (idem est quid ess et esse), as Boethius and Dionysius say; and whether He is, and what He is, is the same (idem est an est et quid est) as Avicenna says. And so He is known through Himself and according to Himself.
But because the quiddity of God is not known to us, in relation to us God’s existence is not known through itself, but requires demonstration. But in heaven, where we will see His Essence, His existence will be much more known through itself by us than it is now known through itself that an affirmation and a negation are not both true at the same time.