Does not this argument prove too much, either that in man too essence and will are the same, or that the human will is an accident to human substance? St Thomas would reply by a distinction between act and faculty. In God, the distinction does not obtain. Act and faculty are one in Him, one with His essence. But in man act and faculty are distinct. The act of volition is accidental in man, it comes and it goes: but the faculty or power of willing is not an accident to human substance, it is in essential property of the same.

Of God and His Creatures: 1.73