In his Summa Theologica, Thomas Aquinas wrote of five ways that the existence of God could be proven. The third way argues from contingency to necessity. In Thomas’ words:
We find in nature things that are possible to be and not to be, since they are found to be generated, and to be corrupted, and consequently they are possible to be and not to be. But is it impossible for these always to exist, for that which is possible not to be at some time is not. Therefore if everything is possible not to be, then at one time there could have been nothing in existence. Now, if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing.
The first significant point is that Thomas starts with observation of the world, not with a mental problem. Things that begin in the mind have a funny problem of not being able to reach beyond the mind to reality. But Thomas starts with observation of reality, then argues from there.
Thomas shows us that we observe things in nature which begin to exist and things that corrupt and disintegrate. Things which are possible to not exist we call possible or contingent. He then shows us that it is impossible for there to be a state of affairs where only contingent things exist, for if this were true, there would be a time where nothing had been generated or everything had corrupted, leaving a situation where nothing existed at all. We find this idea to be true for the following reasons.
Either contingent things existed for infinity or not (this exhausts the possibilities). If they existed for infinity, an infinite series of contingent things would indeed result in a time where nothing existed. Any challenge to this does not truly take into account infinity, for in an infinite series all possible situations would be played out, resulting in a time where something had not been generated or all things had corrupted, leaving nothing, which ends the series. If the contrary was the case, that contingent things have not existed for infinity, then they have been generated. Anything that is generated requires something to exist to generate them, which is the point Thomas is trying to argue toward.
Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another . . . Therefore we must admit the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This men speak of as God.
Here he concludes his third way by reinforcing the point already made, which is that it is impossible for the world to only contain possible things, but there must be a thing that exists which is necessary to exist, and this necessity must be in its own being, not coming from another. Thus Thomas starts with observing the world and argues from possible beings (contingent beings) to the existence of a necessary being, which we call God.
We tend to think of causality in a horizontal series, such as one domino knocking over another domino or two humans generating another. While Thomas’ third way certainly applies to horizontal causality, what is not inherently obvious in Thomas’ argument is that he is also speaking of current, ongoing causality. It is possible for things in the world to stop existing in the next moment, therefore the same series of possible and necessary causality applies to current, ongoing existence.
Of course, all this is much more straightforwardly expressed in the Bible, where Colossians 1:17 speaks of Jesus: “He is before all things, and in Him all things hold together.”