  Aquinas (like Augustine) derived DDI from the deeper classical-theist doctrine of divine simplicity (ST Ia 9). If God is simple, God has no parts of any sort. Now when the turnip aged, it became partly different — its smell and texture altered. Were this not so, no change would have occurred. But if the turnip had changed in every respect, it would not have been a case of change either. For it would have changed with respect to such properties as being a turnip and being identical with this turnip. And if first we have something identical with this turnip and then we have something not identical with this turnip, the turnip has not changed, but disappeared and been replaced by something else. So whatever changes must stay partly the same (else there was not change in one selfsame surviving thing). So only things with parts or distinct aspects can change. If so, a simple God cannot change. DDI's connection with divine simplicity and the classical theist theory of God's perfection which centers on divine simplicity is one of the deepest reasons for DDI's broad historical appeal; one cannot fully explain what moved thinkers to accept DDI without also treating the motivation for the doctrine of divine simplicity. That, however, is too large a topic to broach here.