When thinking about the Second Way of Aquinas, which states that no cause can cause itself and therefore there must be a first cause, we run into a difficult problem of which the Second Way cannot get itself out. This problem is this: Aquinas has only proved this First Cause, he has not proven God.
In light of Modern Cosmology you might make the argument that the first cause was the Big Bang that brought all matter into existence.1 And given the restraints of the Second Way there is no way out of it. You cannot deny that if you define the Big Bang by the simplified way it usually is, then it fits the qualifications of “First Cause”. So then why think the First Cause is God? That is what Aquinas seeks to do in his Third Way. Show that “Even if there is a Big Bang, it did not have to occur, it is possible that the universe could have not existed. Since it is possible for the Big Bang to have not happened, there must be an explanation for the possibility that is namely: the Big Bang did happen.”
The third way is taken from possibility and necessity, and runs 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. 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, as has been already proved in regard to efficient causes. Therefore we cannot but postulate 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 all men speak of as God.
What we have above is a precursor the Leibniz’ Cosmological Argument from Contingency and a follow up on Avicenna’s argument from the same. As his other arguments in this series it depends heavily on the argument against actual infinities, but has a distinct flair from his versions on motion and causation. This is a distinctly modal argument.
While Avicenna drew largely on Aristotelean principles he seems to have come to a different conclusion than Aristotle did. Aristotle and Aquinas both argue that there is a first cause because of an infinite regress of causes is impossible. Avicenna however, argues that since the totality of all possible things cannot be caused by itself (since it is itself possible) it must be caused by something outside itself. And since the thing is outside of the totality of all possible things, it must be necessary. And that means that there is a necessary thing, and that thing is God.
Aquinas doesn’t take this same sort of modal approach to the argument. He argues very similarly to the fashion of Aristotle, namely that the unvierse has to have an explanation of itself. A cause of causes. It is easy to conflate this argument with the Argument from the Beginning of Motion, but we should not do that because they are distinct even though they have a logical similarity to their argument. We should not think of these causes, for example, as temporally sequential, happening one right after the other. Instead we should view them as simultaneous causes: logically simultaneous like what exists in an argument.
Let us take for example his argument:
1. Contingent beings are caused.
2. Not every being can be contingent, for this would create an infinite regress.
3. There must exist a being which is necessary to cause contingent beings.
4. This necessary being is God.
There is not a temporal succession to these things. It is not that ‘First’, step one comes, and then step two and then three and then four. Our minds may track through that path, but they all exist simultaneously. They are on your screen at the same time whether you are reading them or not. And they happen concurrently, and YET there is some degree of dependence of 4) on 3) and 3) on 2) and 2) on 1). If any of these had been different the argument would not work.
In the same way, we like to say that the irrational number Pi, “Goes on to infinity…” which is a misstatement. It is not as if Pi is continuing to count itself out every time it is calculated. It exists in it’s entirety, like a ray but rather all of it’s infinite digits exist simultaneously though each is dependent on the former. This is not to say that it is an actual infinite. Pi has a definite end point, namely the 3 on the other side of the decimal. It is infinite in the future direction only and all later numbers are dependent on that 3. 3 however, could not exist if it were the final number of Pi, that was dependent on an infinite set of variables in the other direction.
Similarly, if I had an argument consisting of infinite premises, all of which was dependent on the others but I came to a conclusion at the end.
Infinite 1: There exists an x
Infinite 2: There exists an x+1
That conclusion would be dependent on an infinite number of non-temporal causes that occurred before it, and therefore could never be reached.
In the same way, Aquinas argues that necessity and possibility work. For every contingent thing must have a cause. To put it another way, every contingent thing is dependent on something else for it’s existence. Since an actually infinite number of causes is impossible, this first thing must be a necessary being.
Since God is the only thing that could exist necessarily (think about Anselm trying to show the absurdity of a God that doesn’t exist) and must exist of its own power, then this first cause is God. It is not just a convenient stopping point for Aquinas it is the end.
- This doesn’t actually help the problem since it doesn’t make sense for that to have occurred un-caused at any point. If there was an infinitely dense ball of matter that existed there since eternity past, there would have had to have been a change in order for it to rapidly start expanding. That would be the first cause. ↩