Thomas Aquinas, in his massive work Summa Theologica, included five logical explanations for the existence of God. If people know anything about Aquinas, they know the five ways. Unfortunately, Aquinas is often misunderstood. A key, but often misunderstood concept is this: Aquinas’ work was not meant to be an apologetic to convince nonbelievers, but was instead a theological explanation for seminary students. As proof, in the very first question of the Summa, before he deals with any other subject, Aquinas addresses the question of whether we can arrive at Christian truth through human reasoning. His answer is a resounding no. Thomas states clearly that attempts to reach God through human wisdom will end in error, and we therefore must rely on revelation from God for Christian truth. Aquinas’ heavy use of logic is often misunderstood as trying to get nonbelievers to reach divine conclusions through human origin, rather than what he is actually doing, which is explaining to seminary students what has already been revealed to us by God.
Further, in Aquinas’ small work Rationibus Fidei: Reasons for the Faith Against Muslim Objections, under the section titled “How to Argue With Unbelievers” he states “First of all I wish to warn you that in disputations with unbelievers about articles of the Faith, you should not try to prove the Faith by necessary reasons. This would belittle the sublimity of the Faith, whose truth exceeds not only human minds but also those of angels; we believe in them only because they are revealed by God.”
Be that as it may, Aquinas’ Five Ways do still hold up logically, as long as they are properly understood. Richard Howe maintains that at least half of introductory philosophy texts misunderstand Aquinas, mainly due to thinking that Aquinas’ use of causality is sequential, when in fact it is simultaneous (or per se).
Aquinas’ third way can be logically diagrammed as follows:
- Some contingent things are existing.
- No contingent things are things that exist for all time and places (assuming an infinite time)
- If all things are contingent then there would be one time where nothing existed (again, assuming infinite time)
- All things that come into existence are things that come into existence by another already existing thing.
- If there was one time where nothing existed then there would not be anything now.
- Therefore it is not the case that all things existing for all time and places are contingent things.
- Existing things are either caused by another (contingent) or not caused by another (not contingent = necessary; law of excluded middle)
- No series that only has caused-causes can come into existence.
- Therefore it is the case that some thing exists that is necessary of itself
(or, not a caused-cause).*
Another criticism is the one that paints Aquinas’ first way as saying everything needs a sufficient reason. But Aquinas’ first way is not using the principle of sufficient reason, as Geisler explains:
The mistake of many theists, especially since Gottfried Leibniz (1646-1716), is to cast he cosmological argument in a context of logical necessity based on the principle of sufficient reason. This ultimately leads to contradictions and an invalidating of the argument. In contrast to this procedure, other theists (like Aquinas) used the principle of existential causality to infer the existence of unlimited Cause or Actualizer of all existence. This conclusion is not rationally inescapable, but it is actually undeniable. In brief, if any contingent being exists, then a Necessary Being exists; if any being with the potentiality to not exist does exist, then a a Being with no potentiality not to exist must exist. (Systematic Theology, 1.577)
So Thomas Aquinas’ cosmological proof for God presented in his Third Way is logically sound and does not depend on the principle of sufficient reason. However, Aquinas does not use it to try to prove to a strident non-Christian that God exists. Rather, he uses it to teach Christian leaders to think through what they believe, and that reason is compatible with faith.
*Thanks to my friend Bruce for assistance with the logical diagramming.