To All: Detailed pickings of Aquinas' First Way #5: The First Way Premise 2

NOTE: THIS IS LESS OF A DEBATE THREAD AND MORE OF AN EXPLANATORY THREAD, FOR EASY LATER REFERENCE. THE AIM OF THIS THREAD IS TO GIVE READERS PROPER UNDERSTANDING OF THE ARGUMENT, NOT NECESSARILY TO DEBATE ITS TRUTH VALUE.

Summa Theologica, First Part, Question 2, Article 3

OK, now we move onto the second premise of the argument. In the last part we looked at the first premise, which basically just says: things are changing. Or to put it into the correct terminology, potentialities are being actualized.

The next thing Aquinas says in the First Way is:

But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.

I.e., To go from potential to actual, something else that is actual is needed. Example: potential ice (actual liquid water) must have actual cold air applied to it to make it into actual ice.

Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold.

What he is saying here is that there is a logical contradiction if something is both potential and actual in the same respect. Potential ice cannot simultaneously be actual ice. It's either one or the other. As a result, the potential cannot make itself actual, as Aquinas illustrates:

It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another.

Since a potential is just a potential, and not actual, then it can't do anything, such as making itself actual. So something else that is actual is needed to make it so.

Important Point

This establishes that if something is mixture of actuality and potentiality, then it requires something else that is actual to change it.

This Concludes Part 5

The premise here is: "whatever is changing (having its potentiality actualized) is being changed by something else (something that is itself actual)." The actual cold air causes the potential ice to become actual ice. What Aquinas is doing here is establishing the "B" in an essentially ordered causal series. The "C" is only happening because a "B" is happening to it. To take the hand pushing a stick pushing a stone example, he is establishing that the stone and stick can't move themselves, and must be pushed by something else.

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