Thomas Aquinas' Five Ways to Prove the Existence of God
from http://guava.phil.lehigh.edu/aquinas/fiveways.html
1. The First Way: Motion.
2. The Second Way: Efficient Cause.
3. The Third Way: Possibility and Necessity.
4. The Fourth Way: Gradation.
5. The Fifth Way: Design.
General Remarks:
* Later thinkers classed all five ways as variants of the cosmological argument for the existence of God. Cosmology is the study of the origins and structure of the universe; each of the five ways is a reflection on the conditions which must have been in place in order for the universe, or some observed feature of the universe, to come about.
* The fourth way looks, at first blush, like a variation on the ontological argument. But like the other four ways, it's a posteriori. Anselm's argument is a priori. It is criticized by Aquinas in Summa I.II.1 (p. 417). Further, says Aquinas (I.II.2), any demonstration of the existence of God must be from the effects of God known to us; it must be a posteriori.
* The fifth way resembles a version of the teleological argument, or argument from design. Though the canonical argument from design is of much later vintage (17 Century), Aquinas might not object to this identification. The teleological argument, after all, is a posteriori.
The First Way: Motion
1. All bodies are either potentially in motion or actually in motion.
2. "But nothing can be reduced from potentiality to actuality, except by something in a state of actuality" (419).
3. Nothing can be at once in both actuality and potentiality in the same respect.
4. Therefore nothing can be at once in both actuality and potentiality with respect to motion
5. Therefore nothing can move itself; it must be put into motion by something else.
6. If there were no "first mover, moved by no other" there would be no motion.
7. But there is motion.
8. Therefore there is a first mover, God.
The Second Way: Efficient Cause
1. Nothing is the efficient cause of itself.
2. If A is the efficient cause of B, then if A is absent, so is B.
3. Efficient causes are ordered from first cause, through intermediate cause(s), to ultimate effect.
4. By (2) and (3), if there is no first cause, there cannot be any ultimate effect.
5. But there are effects.
6. Therefore there must be a first cause for all of them: God.
The Third Way: Possibility and Necessity
1. "We find in nature things that are possible to be and not to be:" contingent beings.
2. Everything is either necessary or contingent.
3. Assume that everything is contingent.
4. "It is impossible for [contingent beings] always to exist, for that which can not-be at some time is not."
5. Therefore, by (3) and (4), at one time there was nothing.
6. "That which does not exist begins to exist only through something already existing."
7. Therefore, by (5) and (6), there is nothing now.
8. But there is something now!
9. Therefore (3) is false.
10. Therefore, by (2), there is a necessary being: God.
The Fourth Way: Gradation
1. There is a gradation to be found in things: some are better (hotter, colder, etc.) than others.
2. Things are X in proportion to how closely the resemble that which is most X.
3. Therefore, if there is nothing which is most X, there can be nothing which is good.
4. It follows that if anything is good, there must be something that is most good.
5. "Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God" (420).
The Fifth Way: Design
1. We observe that natural bodies act toward ends.
2. Anything that acts toward an end either acts out of knowledge, or under the direction of something with knowledge, "as the arrow is directed by the archer."
3. But many natural beings lack knowledge.
4. "Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God" (420).