Does 'nature' mean anything definite and fixed at all? Because, if it does not, nothing can be contrary to nature. Does 'nature' mean merely 'what God wills?' If so, nothing that God wills can be contrary to nature. But the question recurs in another form: 'What can God will? Any fantastic and bizarre combination that we choose to name?' Certainly not. There are then restraints upon God's willing, restraints in the eternal nature of things, which, in the last reduction means God's own nature. His will may be said to be conditioned by His nature. He is not a merum arbitrium, an absolute, arbitrary will. Then there must be something definite and fixed, which may be called 'nature,' against which God can have no will.
Of God and His Creatures: 3.100