II. The Argument from Causality
Cause and effect are apparent in the universe. Everything that occurs is caused by something else. All events are dependent on some other occurrence or thing in order to make them happen. A thing cannot be the cause of itself, or else it would never come to exist. Logically, this chain of causation cannot be infinitely long, or nothing would ever have come to exist in the first place. Therefore, there must be an un-caused thing that causes all other things. This argument is not related to time or a sequence of events. Rather, it considers the fact that all things are dependent on something else for their existence.
In other words, the second of Aquinas’s ways to show God’s existence is based on the fact that all effects are caused by some other event, which in turn is the effect of some other cause. But this chain of causality cannot be infinitely long, so there must be some un-caused cause: God, the First Cause.
III. The Argument from Contingency
Nothing we observe in the universe is necessary; nothing needs to exist, in and of itself. We often observe things that cease to exist, falling victim to death, destruction, or decay. Eventually, all non-necessary things cease to be. But, if it were possible for everything to cease to exist, and if there has been an infinite amount of past time, then all things would have already ceased to exist. There would be nothing left at all. The fact that anything exists at all, even now, means there must be one thing that cannot cease to exist, one thing that must necessarily exist. There must be one thing that is non-contingent—i.e., its existence is not dependent on anything else. This thing must be.
In other words, Aquinas’s third argument or way to prove God’s existence is that, if everything were impermanent, eventually everything would cease to be. Therefore, there must be at least one thing that must, necessarily, exist (one non-contingent thing): God, the Necessary Being.
