Eckhart says: "[Question:] Are Existence and Understanding the Same in God?
I reply that they are the same in reality and perhaps in both reality and in thought.
First I offer proofs I have seen. Five are in the Contra Gentiles and the sixth is in the First Part [of the Summa Theologiae]. All are based on the fact that God is the first and simple being; for something cannot be the first being if it is not simple.
1. First proof: Understanding is an immanent act, and whatever is in the first being is the first being. Hence God is his act of understanding, and he is also his existence. The conclusion follows.
2. There is no accident in God. Hence his existence is the same as his essence. It follows that because God's act of understanding is identical with God and with his essence, it is also identical with his existence. The conclusion follows."