PHL100: Aquinas's Five Arguments


Aquinas's Five Arguments for the Existence of God
Here is just one way to sketch out Aquinas's arguments.

  1. Motion
    • There are things in motion.
    • Things are only actually in motion if they can potentially be in motion, and if something actually moving pushes them.
    • A thing can have a potential and an actuality, but not both at the same time. Something can potentially move, or actually move, but not both.
    • Thus, nothing can push itself. Each thing in motion moved by something else in motion prior to it.
    • [Implicit premise: time is linear.]
    • This chain cannot go back forever.
    • There must have been a first mover.
  2. Cause
    • There are causes and effects.
    • Each effect has a cause.
    • An effect cannot be its own cause.
    • Thus, each effect must have a prior and other cause.
    • [Implicit premise: causal chains are linear.]
    • This chain cannot go back forever.
    • There must have been a first cause.
  3. Possibility and Necessity
    • There are things which are possible: them come into and out of being.
    • By definition, nothing is possible but always existing (here Aquinas seems to mean that possible is a temporal notion, as noted above).
    • Everything is merely possible, so everything could fail to exist.
    • There must have been one time when everything failed to exist.
    • But if there was a time when nothing existed, then there would have been a time when nothing could exist (since nothing can come from nothing).
    • There must exist a necessary being.
  4. Gradition of Being
    • Many (positive) properties come in degrees (some people are more good than others, etc.)
    • More and less are measured against some maximum.
    • There must be something having the maximum properties (all good, all knowing, all powerful).
  5. Governance of the World [The Design Argument]
    • We see that even things without minds or knowledge have goals.
    • These goals are the best result.
    • There must be a design of nature for this to occur.
    • There must be a designer of nature.