It is a metaphysical necessity for God to be infinite, man rational, an animal a living being. Metaphysical necessity is absolute. Physical necessity exists in connection with the activity of the material beings which constitute the universe. But this determination is dependent upon certain conditions, the presence of which is required, the absence of one or the other of them preventing altogether the exercise or normal exercise of this activity. This hypothetical necessity is also called by some consequent necessity.
Moral necessity is necessity as applied to the activity of free beings.
We know that men under certain circumstances, although they are free, will act in such and such a way. This moral necessity is the basis of moral certitude in historical and moral sciences. The term is also used with reference to freedom of the will to denote any undue physical or moral influence that might prevent the will from freely choosing to act or not act, to choose one thing in preference to another. In theology the notion of necessity is sometimes applied with special meaning. Theologians divide necessity into absolute and moral.