Thomas Aquinas, Compendium of Theology

Evidently God's will cannot be anything other than His intellect. For, since a good that is apprehended by the intellect is the object of the will, it moves the will and is the will's act and perfection. In God, however, there is no distinction between mover and moved, act and potency, perfection and perfectible, as is clear from the truths we have already gained. Also, the divine intellect and the divine essence are identical. Therefore the will of God is not distinct from the divine intellect and God's essence.

Another consideration: among the various perfections of things, the chief are intellect and will. A sign of this is that they are found in the nobler beings. But the perfections of all things are one in God, and this is His essence, as we showed above. In God, therefore, intellect and will are identical with His essence.