Catechism 101: Aquinas - proof from possibility and necessity

BY PAT ARENSBERG

“But,” said Moses to God, “when I go to the Israelites and say to them, ‘The God of your fathers has sent me to you,’ if they ask me, ‘What is his name?’ what am I to tell them?”  God replied, “I am who am.”  Then he added, “…I AM sent me to you.”  Exodus 3:14

As we continue to look for ways, using natural reason alone, to arrive at the conclusion that God exists we not turn our attention to St. Thomas Aquinas’ third demonstration, “From possibility and necessity.”  This is likely the most difficult for me to explain in this brief format, so I would like to begin with the conclusion.  Aquinas arrives at the conclusion (again using natural reason) that there must be a being whose existence is not just possible, but in fact necessary.  That being is God.

You see we all have an existence that is possible but not necessary (so does everything in the universe).  We know it is possible for us to exist because we do, and we know that it is not necessary for us to exist because there was a time when we did not.  Everything in the universe is in the same existential boat, namely having an existence that is possible but not necessary.  Aquinas argues that there must be something that has a necessary existence otherwise the universe would not be here.  He says that everything would have fallen into the state of non-existence at the same time and that state would have continued forever.

This is probably my least favorite demonstration in terms of its power to convince, but it is my favorite in terms of insights into God.  He revealed himself as “I am Who am.”  In essence he told Moses 2,700 years before Aquinas, “I am existence; I am the one who must exist.”  I think that is kind of cool.

About Pat Arensberg Patrick Arensberg is the Director of Religious Education for the Archdiocese of Mobile. Previously, he taught for 17 years at McGill-Toolen Catholic High School, where he served as Chairman of the Theology Department. He attended the Gregorian University in Rome and holds an M.A. in Theology from Notre Dame Seminary in New Orleans. He is married to Connie and they live in Mobile with their 5 children.