On this blog there have been long and intense discussions among Catholics, agnostics, and atheists that either point to, or directly involve, the logical proofs of God’s existence. Here is a scaled-down version of the Unmoved Mover proof that St. Thomas Aquinas wrote in the Summa Contra Gentiles. I am not suggesting that any words should be altered, but rather attempting to pull out the key points so someone new to reading his writing has a map through the argument.
The argument is in Book One, Chapter 13, Sections 1-19 (massive volumes of writing come before and after it). Whether you are a believer or non-believer, try to look on these proofs objectively. Do you remember marveling at shapes as a child, noticing the symmetry and connection between circles, triangles, and squares? Do you remember learning about the principles of geometry, or especially trigonometry, as a student? You sensed something could be put into words, and although the proofs were rigorous, when you realized how the relationships fit together, it was exciting (if not daunting). Approach these proofs that way. St. Thomas references them all to Aristotle’s Physics, and before you think you’ve found the “Gotcha!” error in either scholar’s reasoning, please consider that these folks were also believers. Click the link? Good. Now without further adieu…the argument.
Everything that is moved is moved by another, in movers and things moved one cannot proceed to infinity.
“Everything that is moved is moved by another. That some things are in motion—for example, the sun—is evident from sense. Therefore, it is moved by something else that moves it. This mover is itself either moved or not moved. If it is not, we have reached our conclusion—namely, that we must posit some unmoved mover. This we call God. If it is moved, it is moved by another mover. We must, consequently, either proceed to infinity, or we must arrive at some unmoved mover. Now, it is not possible to proceed to infinity. Hence, we must posit some prime unmoved mover. Both statements can be proved.” (Section 3)
Everything that is moved is moved by another. Proved in three ways:
1) Whatever is moved is divisible (Aristotle Physics VI, 4). Moving things must be divisible, must have parts. Why? Because to be moving or changing, the same thing cannot be both unchanged and changed all at once. Aristotle wrote this before we knew of atomic structure, but consider what we’ve learned since. Everything that has been discovered is further broken into parts, and is constantly changing. If you move your arm, it is moved by both arm muscles and other muscles, which are moved by muscle cells and other cells, which are moved by molecules, which are moved by atoms, which are moved by atomic particles, et cetera.
So, for something not to be moved by another thing, the moving thing would have to primarily move itself, be moved by reason of itself, not by reason of a part of itself. It would have to be, as a whole, at rest, and then, as a whole, move. If a part were at rest the whole would be at rest, because there would be no parts. Since moving things must logically have parts, this is a logical impossibility.
This is called a conditional proposition. St. Thomas gave the example, “If man is an ass, he is irrational.” Man cannot be an ass (stop snickering, he means the animal), nor can he be irrational (possessing an irrational soul), but if he were an ass he would necessarily also be irrational.
2) Whatever is moved by accident is not moved by itself (Aristotle Physics VIII, 4). “Accident” means a property or quality not essential to a thing. To be moved by violence, means to be moved unnaturally by another. So things that are not animals that move (rocks) must be moved by another, since the movement is by accident. This is a proof by induction.
3) And to return to divisibility, since things that move are divisible, the same thing cannot be both action and potential (Aristotle Physics VIII, 5). Thus nothing can be both mover and moved, and therefore, nothing moves itself. Logically impossible.
In movers and things moved one cannot proceed to infinity. Proved in three ways:
1) If all movers and things moved proceed to infinity, then there is no succession. All infinities move together. If one of them is finite, i.e. moved in a finite time, then all the infinites are moved in finite time. This is impossible. The mover and the thing moved must exist simultaneously, which would mean all things move as one single mobile, and one infinite is moved in finite time, which is, again, logically impossible (Aristotle Physics VII, 1).
2) Or, in an ordered series of movers and things moved in succession (a series where one thing is moved by another), but a succession that proceeds to infinity, there still must be identified a first mover. Why? Because if there is no first mover, there is no thing moved. If the first mover is removed, or ceases to move, no other mover will move or be moved. The first mover is the cause of motion for all the others. There can be no infinite series of intermediate movers, it is a logical impossibility.
3) Or, reverse the order. That which is moved cannot move unless there is a principal moving cause. Nothing will be moved.
Still Not Clear?
If this is not clear, study (as opposed to skim) St. Aquinas’ or Aristotle’s writing. In this chapter Aquinas goes on to address the next question, “Can the Unmoved Mover move?” He answers no, referencing back to the logical problems above, and including the arguments against it from others. In the diagram, the reason the second “No” points to God is because the thing is also not moving (it wasn’t moved), and the third “No” infers that there must be a First Unmoved Mover.
This is enough for now, so let the discussion begin. Conversion of heart is a matter of will, and while we pray for that continual conversion for ourselves and for others, this logical exercise is about knowledge. Not every trigonometry student loved mathematics when he began to study the discipline either.