To Florus 5.45

But just as truth has exposed and overthrown you both, so consideration of our duty requires us to explain what made this now-disarmed question obscure. Everything that occurs is said to arise either from necessity or from possibility. By ‘necessary’ here I mean not what we ordinarily call useful, but what is compelled by stronger causes. We therefore call necessary not what lies within the will’s authority, but what is subject to a force requiring its existence. We call possible what undergoes no necessity on either side, either to exist or not to exist, but in definite ways can exist and can fail to exist. Let our reader hold firmly to what we mean here by necessary and possible.

To begin with great examples: God’s making the world arose from possibility, not necessity. That is, his almighty power made it possible to create what he formed, but not necessary. No one compelled him to make it; he made it because he willed to, and certainly would not have made it had he not willed to. Yet what belonged to the maker as a possibility became a necessity in the work. The world, commanded to exist by the Almighty, did not have the possibility both of existing and not existing: it was compelled to exist, since the Almighty commanded its being.