Thomas Aquinas: Arguments for God’s Existence

Kinds of Arguments and the Argument from Motion

January 14, 2004

Overview

A priori and a posteriori arguments. Why Prove God’s Existence? The Arguments

Argument from Change Argument from Causation Argument from Contingency Argument from Degrees of Excellence Argument from Harmony

Next Time

Just as it says on the syllabus: Do SQ 4, p. 49 Do FFR 3, p. 51

A Priori and A Posteriori Arguments A Priori Arguments

Premises don’t require any “empirical” information. Instead, the argument proceeds purely conceptually. Ex: All bachelors are unmarried men, and no unmarried

men have wives. Therefore, no bachelors have wives. A Posteriori Arguments

Premises do require “empirical” information. They require particular, contingent information about

how the world is, beyond just conceptual information. Ex: If the streets are wet, then my basement is flooded.

But my basement is not flooded, so the streets are not wet.

Why Prove God’s Existence?

There are some reasons for doubting God’s existence. The Problem of Evil Occam’s Razor

The Argument from Change

‘Motion’ is used in its general sense, to mean “change.”

What is the basic idea of this argument?

The First Way Simplified

1. Things move and change.2. Whenever something moves or changes, its motion or

change must be caused by some other thing, not by itself or by nothing.

3. If that other thing is also moving or changing, then its motion or change must be caused by some further thing still.

4. If the chain of causes of changes stretched back infinitely, it would have no first link to get it started.

5. So, the chain of causes of changes does not stretch back infinitely.

6. So, there must be some First, Unmoved Mover, namely God.

What’s All This “Potentiality” Stuff? It all depends on a lot of medieval philosophy

you don’t know. But, we can fix that a little bit:

A “property” is a feature of something: redness, hotness, blandness, perfection, etc.

To change is to gain a property or to move to a different place on some scale.

Think of properties as contagious: I can’t gain a property without the intervention of something that already has at least as much of it as I wind up with.

This means nothing can cause changes in itself – changes have to come “from the outside.”