There is a principle of causality that states, in its simplest form, that every effect has a cause. This can be challenged by those who claim the problem of induction, namely that we have not observed every effect being generated, therefore we cannot make the conclusion. While it is true that we have not observed every effect, attempting to deny the principle would involve saying that we cannot be sure that every effect has a cause, a line of reasoning that ultimately ends in the absurdity of radical skepticism, saying that we know that we cannot know. The skeptic could merely claim that we have not observed every cause and effect, therefore we cannot be sure of the principle of causality, claiming that the principle has not been proven. But the skeptic has proved nothing himself, but merely laid out the possibility. Lastly, denying the principle of causality is a position that cannot be held consistently or lived in practicality. We cannot go through a day of our lives not being sure whether when we walk across the floor to get a drink of water that the floor will be there like it was last time, or whether the water will miss our lungs and flow down our throat like it always has before.
The principle of causality becomes relevant in several areas, one of which is the argument from contingent beings to a necessary one. Thomas Aquinas’ explained it thus:
We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is 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. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence—which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary.
Aquinas is saying that every object we observe in nature was generated, has some corruption, and is possible for it not to exist, a situation known as contingency. If it were true that everything in existence were contingent, then at some point, given enough time, there would be a situation where nothing existed, a state of affairs that would still exist now, for absolute nothing produces no things. Further, as Geisler explains:
All contingent beings need a cause, for contingent being is something that exists but that might, under other circumstances, not exist. Since it has the possibility not to exist, it does not account for its own existence. In itself, there is no reason why it exists. Once it was nonbeing, but nonbeing cannot cause anything. Being can only be caused by being. Only something can produce something.
Note that we are not saying everything needs a reason for its existence, but a cause. If all that exists is corrupt, contingent, and needs a cause, nothing would now exist, a situation that is clearly false. Therefore a necessary being must exist.
This necessary being cannot be made of matter, for all matter has some contingency and need for a prior cause. Attempting to deny this falls prey to the problems explained in the first paragraph. Denying the contingency of all matter either ends in absurd skepticism or proves nothing.
Therefore a necessary, non-material being must exist. This all men call God.