6. Summa Contra Gentiles By Thomas Aquinas Summary Book 1 Chapter 13

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Chapter 13

Arguments in prof of God’s existence

Thomas has already shown that proofs of God’s existence are not futile.   He starts then to prove this through Aristotle’s route.
1.   Whatever is in motion was set in motion by something else.  Whatever moved it either was moved itself or not.   If it was moved, the mover was itself moved or not.  And so on.  This will proceed to infinity in and an infinite regress problem.  It is not possible to proceed to infinity (infinity being b******t).  Therefore we must come to an unmoved mover, the source of all movement. (Aristotle and Aquinas call all change movement.)

This poses two problems.  1. Is motion moved and 2. The problem of infinity.

Aristotle proves 1 by showing that if something moves by itself it would contain movement in and of itself, otherwise it would be moved by something else.   It would have to move because of itself as a whole.  If it’s foot moved, something would have to move the foot and so on, making that thing divisible.  Those divisible parts would then have a regress problem, with the motion regressing to a cause outside of the thing.  The mover moves because of itself.

We do not need to believe that if it moves a part of it can be at rest.
This mover would move independent of all other things.  It is indivisible into parts.  

Aristotle also proves this (a weaker inductive proof) by observing that all things in nature move by either force/accident, or by their Aristotelian souls.   (See Aristotle’s De Anima).  Things either move by something in them, like animals, or something outside of them, like falling bodies.

Thirdly, he says that nothing is at the same time acting and potentially acting IN THE SAME RESPECT. (See Aristotle’s Metaphysics for “Act” and “Potency”).   Whatever is moving is in potency because motion is the act of something that has potency.   That moving billiard ball  can bang into another ball, causing the second ball to move.  The first ball’s motion is potency, or potential, for further motion.  The motion of the first ball is the result, or ACT, of the cue stick.

Aristotle shows in book 6 of physics that movement is the act of something that has potency.   For Plato, movement could include thought.  In terms of the first mover Aristotle and Plato are in agreement.  The first mover understands, desires, and loves itself.    Plato would argue that the mover moves itself.  Aristotle that it is unmoved and unmovable.

Moving on, Aristotle solves the infinite regress problem.  The mover and the moved act simultaneously. The billiard balls touch and in that instant the potency becomes act, etc.   Aristotle tells us that his observation is that mover and moved must be either continuous or in contact. This necessitates them having bodies.

Therefore, if there were an infinity of movers, having bodies, all movers being corporeal, they would have to move in a discrete time.  The infinity of movers each taking a moment to move would require infinite time, which cannot be infinite.

Next, in a chain of movers and moved, if the first mover is removed or ceases, none of the others in the chain will move.  If the chain of movement is infinite there will be no prime mover and the chain cannot move.  Movement from an infinite chain is impossible.

Taken in reverse, anything that moves must take movement from something before, all that moves will be moved movers, therefore there will be no principle mover.  Nothing moves.

What comes before are Aristotles demonstrations FOR a first mover.

The other way to prove this follows.    If every mover is moved this statement is either true in itself OR it is true by accident or coincidence, in which case the statement is not necessary.   This makes a contingent proposition that no mover is moved.  If that is the case it does not move, so NOTHING moves.  It is impossible that there is no movement.  Therefore the statement is not an accident, but is true in itself.

Also, if two things are found accidentally (not in terms of randomness, but in terms of metaphysical accidents, or non-essential characteristics) in a subject it is probable that the two accidents are found without the subject.  So if Socrates is white and musical, being non-essences, or accidents of Socrates-ness those accidents will be found elsewhere, like in Thomas Jefferson, white, and Miles Davis, musical.

It follows then that if mover and moved are in a subject, being accidents it is probable that a mover can be found that is not moved.

If the mover is moved something weird follows, that the motion must be created by the same kind of motion, so the healer must be healed and the teacher must be taught.  Inconceivable as the teacher must have science to teach and must not have it to be taught.  This cannot be.

If movement is created by another kind of movement, we cannot go on indefinitely because there are not infinite kinds of movement.  This, too, cannot be.

The prime mover argument solves this.  The first mover is immovable.  It must be immovable in order to reconcile the problems already described AND it cannot move because it has no part.  Having no part, it is perfect (complete, lacking no-thing) and everlasting.

Some things move through no will of their own.  Digestion ultimately moving animals, for example. This is accidental and not essential.   The self-mover in this model is always in motion in order that movement can be everlasting.  This argument ultimately leads us to acknowledge that the first self-mover is moved by an unmoved mover.

This idea is not rebutted by the argument that the movers of lower spheres (Ptolemy) is everlasting and moved by the lower objects within the spheres.

God is not part of the self-mover.  The philosopher in Metaphysics shows us that the unmoved mover is separate and moves the first mover.  All self-movers move on account of appetites. (For Aristotle the appetites aren’t about food.  The appetitive soul govern desire and virtue.  The other components of the soul are the nutritive and the rational.  In De Anima Aristotle attributes movement, change, etc. in living creatures to one or more of these three aspects of the soul.  Aquinas is telling us that it is the appetitive desire for union with God, the beatific vision, that ultimately moves the first mover, and by extension, us.)  The first mover desires the unmoved mover which is separate and is God.

The idea of eternity of movement messes this up, because it is false. The world and movement had their beginning with the unmoved mover.  We know this because nothing evolves non-being to being.

Another problem is with the assumption that the heavenly body has motion in and of itself. (Ptolemy again I think.)  If the first mover takes it’s motion from the unmoved mover solves all of the problems attendant with this inborn heavenly motion idea.

It is impossible for contrary and discordant things to accord in one order always or frequently except by someone’s governance, through which each and all are made to tend to a definite end.  Now we see that in the world things of different natures accord in one order, not seldom and fortuitously, but always of for the most part.  Therefore, it follows that there is someone by whose providence the world is governed.  And this we call God.


My commentary is in parenthesis.

I am reading the Fr. Laurence Shapcote, OP translation.  I have the opera Latin/English 2 volume set from the Aquinas Institute.  You can buy it here.   I think you can find it in less expensive out of copyright editions if you look for the Dominican Friars Translation on Bezos’ site, or for free at archive.org.

I’m reading about 35 chapters per month. It’ll take about two years.  A few pages a day will get it done.   Join me.

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