Their existence comes to them from an external efficient cause. And so in relation to existence, God alone is absolutely necessary, all others are contingent. When we consider the divers beings, not from the point of view of existence, but in relation to their constitution and activity, necessity may be classified as metaphysical, physical , and moral. Metaphysical necessity implies that a thing is what it is, viz. 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.