We know from experience that things change. Something that is potentially one way becomes actually that way.

A cold thing can become hot, it has the potential to be hot. Something at rest has the potential to begin moving. A seed has the potential to become a tree. Therefore, we can conclude by everything around us that change exists.

But what exactly is happening when something changes?

What do we mean exactly when we say change exists? A thing changes when a potential is actualized, or brought into actuality.

A potential is way something can be when it is not yet actually being. Actuality is what something is here and now.

Something potentially hot becomes actually hot. Something potentially moving 1becomes actually moving. Aquino’s uses the example of fire causing wood that is potentially hot to become actually hot.

Something cannot be both potential and actual in the same respect at the same time

The wood can be actually cold while being potentially hot. But its’ potential to be hot cannot, by itself, make it actually hot. Therefore, whatever is being changed must be actualized by something that is already actual.

The thing doing the actualization may itself be undergoing change and therefore depend upon something else that is already actual. The fire makes the wood hot, but the fire itself may depend upon something else.

This gives us a series of things whose ability to actualize something depends upon something else. But what kind of series is this?

This isn’t about what happened in the past. The concern is with an essentially ordered series: a series in which the causal activity of one thing depends upon another here and now.

One example would be of a hand moving a staff. The staff moves because the hand is moving it. The hand’s causal activity is itself dependent upon something else. Another example that may help is a phone that has the potential to be charging, a cable with he potential to charge, and an outlet with the potential to give power to something else. The phone would be actually charging by the cable that is actually charging the phone by being plugged into the outlet that is actually powered, and so on. Thus, the phone is charging here and now.

So, here’s the key. The question isn’t simply, “What was the first event?”

The question is “What ultimately accounts for the causal activity taking place right now?”

If every actualizer in an essentially ordered series depends and requires upon another actualizer, there would ultimately be nothing that could account for the actualization taking place1. Therefore, the series must terminate in something that is not itself being actualized by another.

In other words, there must be something that is fully actual. This means that something must exist with no unrealized potential requiring another thing to actualize it. It has no potential, but is fully actual. Aquinas would refer to this something as the unmoved mover.

We have reached a reality that is fully actual. It has no unrealized potential and does not need to be actualized by anything else. This means it cannot depend upon something to make it what it is. It is not changing from potentiality to actuality, because there is no unrealized potential within it.

A reality that is fully actual, independent, and the ultimate source of actualization has the characteristics of what we mean when we speak of God, a being that is fully actual.

Aquinas therefore identifies the unmoved mover, the fully actual reality at the foundation of change, with God.