Whether God's Existence can be Demonstrated
We proceed to the second article thus:
1. It seems that God's existence cannot be demonstrated. God's existence is an article of faith. But matters of faith cannot be demonstrated, since demonstration makes a thing to be known, whereas the apostle makes it clear that faith is of things not seen (Heb., ch.11). It follows that God's existence cannot be demonstrated.
2. Again, the medium of demonstration is the essence. But as the Damascene says (1 De. Fid. Orth.4), we cannot know what God is, but only what he is not. It follows that we cannot demonstrate that God exists.
3. Again, God's existence could be demonstrated only from his effects. But his effects are not proportionate to God himself, since God is infinite while they are finite, and the finite is not proportionate to the infinite. Now a cause cannot be demonstrated from an effect which is not proportionate to itself. It follows that God's existence cannot be demonstrated.
On the other hand: the apostle says in Rom.1:20: |the invisible things of him . . . are clearly seen, being understood by the things that are made.| Now this is possible only if God's existence can be demonstrated from the things that are made. For the first thing that is understood about anything is its existence.
I answer: there are two kinds of demonstration. There is demonstration through the cause, or, as we say, |from grounds,| which argues from what comes first in nature. There is also demonstration by means of effects, or |proof by means of appearances,| which argues from what comes first for ourselves. Now when an effect is more apparent to us than its cause, we reach a knowledge of the cause through its effect. Even though the effect should be better known to us, we can demonstrate from any effect that its cause exists, because effects always depend on some cause, and a cause must exist if its effect exists. We can demonstrate God's existence in this way, from his effects which are known to us, even though we do not know his essence.
On the first point: the existence of God, and similar things which can be known by natural reason as Rom., ch. i, affirms, are not articles of faith, but preambles to the articles. Faith presupposes natural knowledge as grace presupposes nature, and as perfection presupposes what can be perfected. There is no reason, however, why what is in itself demonstrable and knowable should not be accepted in faith by one who cannot understand the demonstration of it.
On the second point: when a cause is demonstrated by means of its effect, we are bound to use the effect in place of a definition of the cause in proving the existence of the cause. This is especially the case with regard to God. For in proving that something exists, we are bound to accept the meaning of the name as the medium of demonstration, instead of the essence, since the question of what a thing is must follow the question of its existence. Since the names applied to God are derived from his effects, as we shall show in Q.13, Art. i, we may use the name |God| as the medium in demonstrating God's existence from his effect.
On the third point: effects which are not proportionate to their cause do not give us perfect knowledge of their cause. Nevertheless, it can be clearly demonstrated from any effect whatever that its cause exists, as we have said. In this way we can prove God's existence from his effects, even though we cannot know his essence perfectly by means of them.