God's own being is not only conformed to His intellect, but His act of understanding is the measure and cause of every other being and of every other intellect, and He is Himself His own existence and act of understanding. Whence it follows not only that Truth is in Him, but He is Truth itself, and the Sovereign and First Truth.
SAINT THOMAS AQUINAS Summa Contra Gentiles, i, q. 16, a. 5.