Does God Know He Is Not a Brain In a Vat?
There is the question of if and how God knows he is not a brain in a vat. The answer is yes, God knows that he is not a brain in a vat; however, the more interesting question is how. We can know God knows he is not a brain in a vat because we can know God is not a brain in a vat (by causal reasoning to God as pure subsistent existence itself; nothing exists apart from God unless God causes it, including mad scientists probing brains in vats; also, because God is immaterial, etc). Put that in with the fact that anything we can know God can know better, and God knows he is not a brain in a vat. Further, given that God knows he is the fullness of existence as such and that nothing can exist except for God causing it, God can know (with certainty in knowing himself) that he is not a brain in a vat, deluded by some demon, wired up in the matrix, or whatever.
Remember: There is strict identity between God’s act of understanding and the object of God’s understanding (the divine essence). So, the question of how God knows he is not a brain in the vat is either as clear or as mysterious as the question of how God knows anything. The answer is simple: God knows it in virtue of knowing himself perfectly, in virtue of there being no distance or difference between God’s act of understanding and God’s object of understanding, which is the divine essence – subsistent existence itself. God does not come to truth because God simply is truth.
As Aquinas writes, “…truth is found in the intellect according as it apprehends a thing as it is, and in things according as they have being conformable to an intellect. This is to the greatest degree found in God. For His being is not only conformed to His intellect, but it is the very act of His intellect; and His act of understanding is the measure and cause of every other being and of every other intellect, and He Himself is His own existence and act of understanding. Whence it follows not only that truth is in Him, but that He is truth itself, and the sovereign and first truth.” (Source.)
Again, we can know that this must be the case even if we do not know how this is the case; likewise, can know that there is some way how God knows the things he does even if we cannot know that way ourselves. Thomists love their principle of analogy.