Friday, October 18, 2013

Aristotle's Proof from Motion

Most of the ancient Greeks believed that the existence of the gods was self-evident, so they did not bother to spend time trying to prove their existence.

The scientist-philosopher Aristotle offers a proof for the existence of God in his seventh physics book.  In this book, he talks about motion and inertia; he starts by proving the claim "everything that is in motion must be moved by something."  In modern physics, we call this Newton's First Law of Motion.  You might remember these from school:
  • An object that is at rest will stay at rest unless an external force acts upon it.
  • An object that is in motion will not change its velocity unless an external force acts upon it.
Aristotle gives three arguments to prove "everything that is moved is moved by something else".  I could discuss these, if you readers want me to.  I don't think it's necessary to prove it, because modern physics agrees with Aristotle's Physics on this.

Okay, so everything is either at rest or in motion, unless an external force acts upon it.  Naturally, the external force itself must either be at rest or in motion.  This gives us two possible scenarios for the existence of movement:

1. There is an infinite sequence of things which move other things.
2. The sequence of things which move other things has a starting point, or a first movement.

The second situation is the one we're interested in.  Logically speaking, the first movement must be caused by something immobile; otherwise, it's just another step in the sequence of things which move other things.  This immobile mover is what we call "God", who can create from nothing and move things without being moved.

Aristotle gives three reasons why Scenario #1 is incorrect, meaning Scenario #2 has to be true.  Again, modern physics agrees with Aristotle that there is a starting point where all motion comes from, i.e. the big bang.  Here's his reasoning for why movement is not generated by an infinite sequence.

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1. Some ancients believed the universe was a perpetual motion machine, like a giant game of "ring-around-the-rosie" in which a group of children hold hands and run around in a circle.  This scenario allegedly generates infinite movement, because each child is being pulled, at the same time that the child is pulling someone.  Note that all these motions must be simultaneous, or the system falls apart.

It's impossible to get infinite movement in this scenario, because each individual movement is finite.  You get stuck in a scenario of infinite motion in finite time.  "And since the motion of A and that of each of the others are simultaneous, the whole motion must occupy the same time as the motion of A: but the time occupied by the motion of A is finite: consequently the motion will be infinite in a finite time, which is impossible."

This is actually a double proof, because infinite motion is impossible in finite time and in finite movement.

2. The other ancient hypothesis for infinite movement is a system that has ordered movement, not simultaneous movement.  In other words, the system has an ordered series of movers and things that are moved.  You can think of this as a series of dominoes or as a Rube Goldberg machine.

That scenario doesn't work, when you consider the intermediary dominoes.  They don't move on their own; they all depend upon the first domino for their movement.  Take away the first domino, and the entire sequence is stopped.

Well, in an infinite series of dominoes, every single domino would fall under the category of "intermediary mover".  Hence, none of them move on their own.  Hence, nothing in the system would ever move.

3. Similarly, the scenario falls apart when you consider how secondary movers work.  A secondary mover always follows a primary mover.  If the imaginary infinite series of dominoes, you have nothing but secondary movers, without a primary mover.  Hence none of them move, and the system generates no movement.

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