CT 3, The Existence of God

The first two chapters of the Compendium Theologiae introduce the work and describe its structure. The meat begins in chapter 3. Thomas begins,

Regarding the unity of the divine essence, we must first believe that God exists. This is a truth clearly known by reason. We observe that all things that move are moved by other things, the lower by the higher.

The verb to move in this context means to change. We’re not solely talking about motion from place to place, but about all kinds of change.

The elements are moved by heavenly bodies; and among the elements themselves, the stronger moves the weaker; and even among the heavenly bodies, the lower are set in motion by the higher.

Thomas’ work was a synthesis of Christian revelation with Aristotelian cosmology, which is where this comes from. By elements, I gather he means the four elements of earth, air, fire, and water; and in his day the heavenly bodies, e.g., the Sun, the Moon, the Planets and the Stars, were thought to be different in kind than our own earth. However, as this is given more as an example than as part of the argument I think we’re OK to move on.

This process cannot be traced back into infinity. For everything that is moved by another is a sort of instrument of the first mover.

An instrument, then, is essentially a tool: something I move—that is, something I use—in order to move something else. What he’s saying here is that if A moves B, and B consequently moves C, then B is, in a sense, A’s instrument.

Therefore, if a first mover is lacking, all things that move will be instruments. But if the series of movers and things moved is infinite, there can be no first mover. In such a case, these infinitely many movers and things moved will all be instruments. But even the unlearned perceive how ridiculous it is to suppose that instruments are moved, unless they are set in motion by some principal agent. This would be like fancying that, when a chest or a bed is being built, the saw or the hatchet performs its functions without the carpenter.

Don’t confuse this with the argument from design that the presence of a watch implies a watchmaker. This is simply saying that we mustn’t ascribe to B the effects ultimately caused by A.

Accordingly there must be a first mover that is above all the the rest; and this being we call God.

And if we follow the change of causes all the way back, one of two things happens: either we follow it infinitely (which Thomas would not admit) or we arrive at some mover G which moves other things but is not an instrument of any third thing. And G we call God.

So as Thomas points out, we can actually prove God’s existence using pure reason. But we still haven’t proven that G has all of the other attributes we believe God to have. This is just the first step.

Update: I should point out that Thomas gives five distinct arguments for God’s existence in the Summa Theologiae, and I gather he goes into even greater detail in the Summa Contra Gentiles.  The Compendium is Thomas’ version of Theology for Dummies.

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