We proceed to the fourth article thus:

1. It seems that essence and existence are not the same in God. If they were the same, nothing would be added to God's existence. Now the existence to which nothing is added is the universal existence which is predicable of all things. Hence God would be the universal existence which is predicable of all things. But this is false, according to Wisdom 4:21: "they gave the incommunicable name to stones and wood." It follows that God's essence is not his existence.

2. Again, it was said in Q.2, Arts.2 and 3, that we can know that God exists. But we cannot know what God is. Hence God's existence is not the same as what he is, or his quiddity, or nature.

On the other hand: Hilary says: "Existence is not an accident in God, but subsisting truth" (De Trin.7).

I answer: God not only is his essence, as was shown in Art.3, but also is his existence. This can be shown in many ways. First, whatever a thing possesses in addition to its essence must either be caused by the principles of its essence, as is a property which is consequential to a species, such as laughing, which is consequential to "man" and caused by the essential principles of his species; or it must be caused by something external, as heat in water is caused by a fire. Hence when a thing's existence is different from its essence, its existence must either be caused by the principles of its essence, or be caused by something external. Now a thing's existence cannot possibly be caused by the principles of its own essence alone, since nothing can be the sufficient cause of its own existence, if its existence is caused. Hence anything whose existence is different from its essence must be caused by something other than itself. But we cannot say this of God, who is denned as the first efficient cause. It is therefore impossible that God's existence should be different from his essence.

Secondly, existence is the actuality of every form, or nature. That is, we do not say that goodness or humanity, for example, are actual, unless we mean that they exist. Hence where essence and existence are different, existence must be related to essence as the actual to the potential. But it was shown in Q.2, Art.3, that there is nothing potential in God. It follows that essence and existence are not different in God. God's essence, therefore, is his existence.

Thirdly, anything which has existence without being existence exists through participation, just as anything which is alight but is not itself fire is alight through participation.

Now we proved in Art.3 that God is his essence. It follows that, if God were not his own existence, he would exist not through his essence but through participation. But God would not then be the first being, which is an absurd thing to say. God is therefore his own existence, as well as his own essence.

On the first point: "that to which nothing is added" may mean two things. It may mean that a thing's nature precludes the addition of something. The nature of an irrational animal, for example, excludes reason. But it may also mean that a nature does not necessitate the addition of something. Thus the common nature of animal does not have reason added to it, because it does not necessitate the addition of reason, though neither does it exclude reason. It is in the first sense that nothing is added to God's existence, and in the second sense that nothing is added to universal existence.

On the second point: "is" may signify two things. It may signify the act of existing, or it may signify the synthesis by which the mind joins a subject to a predicate in a proposition. Now we cannot know the divine act of existing, any more than we can know the divine essence. But we do know that God "is" in the second sense, for we know that the proposition which we put together when we say "God exists" is true. We know this from his effects, as we said in Q.2, Art.2.