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Thomas Aquinas' Third Way

SUMMARY

If one observes the world for any length of time, one realizes that we wear out. People grow old, cars rust, and animals of all kinds come into being and then cease to be. This kind of change is so common that most people do not take time to reflect on what it implies about the world. This fact about the world shows that the world is full of beings that are corruptible. If the beings of our world are corruptible, they have in themselves the ability to not be.

Upon examination of this fact, we see that we are possible beings. Our existence is not necessary, for we all know that our death is a very real possibility. If we are only possible beings, there must be something that caused our existence, for possible beings cannot cause their own existence. Further, if all beings were possible beings, there would be no beings at all.

Further, it is impossible for possible beings to cause possible beings because possible beings only have existence; they are not existence itself. Only a being that is existence can give existence. Possible beings can only receive existence. Since possible beings cannot be the cause of possible beings, there must be a necessary being who is the cause of possible beings. This necessary being is God.

A step by step summary of this argument is as follows:

1. Corruptible beings exist.
2. Corruptible beings are possible beings. Their existence is not necessary.
3. If only possible beings existed, there could be no possible beings.

a. Possible beings require a cause of their existence.
b. Possible beings cannot be the cause of their own existence.

4. However, there are possible beings.
5. Therefore there must be a necessary being that is the cause of all possible beings.
6. This necessary being is God.

PREMISE 1

The first premise is derived from the empirical fact of corruption. Things in the world corrupt; therefore, things in the world must be corruptible. Corruptible beings have within themselves the possibility of corruption. It is not the case that corruptible being is incorruptible being that is merely exposed to some external destructive force that corrupts it. Rather, corruptible being is the kind of being that, because of what it is, has the ability to corrupt.

PREMISES 2 AND 3

The second premise states that corruptible beings are beings that have the possibility not to exist. In other words, possible beings begin to exist and are able to cease to exist. If a being comes into existence and is able to cease to exist, then its existence must be possible. Further, that being must have a cause of its existence. If a being’s existence is not necessary, then we must ask why that being exists at all.

There is, however, a strong objection to this line of reasoning. It may be said that human beings, which are possible beings, are able to cause the existence of other human beings. In fact, all living things reproduce and are responsible for the existence of other possible beings. It would appear then that the fact of biological reproduction defeats the claim that possible beings cannot cause possible beings. And if possible beings can cause other possible beings, there is no need to seek a further cause for the existence of possible beings.

In response, biological parents are not the efficient causes of the child’s existence. Rather parents are the instrumental cause of their child’s existence. All the parts that make up a child already exist. Parents are not the cause of the child’s existence, they are the cause of a substantial change from a non-human being to a human being. Biological creatures merely change existing things. They do not account for the existence of the things they change.

Beyond that, we may legitimately ask what caused the existence of the parents. If we respond by appealing to the parents of the parents, we have merely succeeded in pushing the question back one step further. But why do we even seek an explanation for the existence of the child? Isn’t it because we know that people cannot account for their own existence? We always appeal to something which is not the being itself to explain the existence of the being in question. So even the individual who supposes that parents can account for the existence of the child realize that people do need a cause of their existence.

Since we know that people need a cause of their existence, and that other people cannot be that cause, it follows that something else must be the cause. Materialists often appeal to matter as the ultimate cause for the existence of biological beings. According to the materialist, no further explanation is needed for the existence of material things. Matter is as far as the causal chain goes.

At first, this appears to be a viable answer since all biological creatures are made of matter. Biological creatures may not be able to account for their own existence, but perhaps matter can. Matter does appear to be able to account for the existence of animate and inanimate beings, since all material beings are made of matter.

However, matter itself, if understood as atoms, is able to change. It has within itself the possibility of corruption. As noted above, any being that has within itself the possibility of corruption is possible being. Possible being requires an explanation for its existence. What we find then, is that the supposed explanation for the existence of all things requires an explanation for its own existence.

PREMISE 4

The fourth premise affirms that possible beings do exist. This premise is constantly confirmed by experience. The only way to deny this premise is to affirm that the world of possible beings is illusory. However, even those who think that the world is illusory stay off the train tracks when trains are coming. You may affirm that the world is illusory, but no one really lives that way. This suggests that even those who hold this sort of idealism do not really give themselves completely to their own view. The reason they do not give themselves completely over to this view is because they have a constant reminder, via experience with the world, that their view is wrong.

PREMISE 5

Given the fact that possible beings exist, that they require a cause for their existence, and that they cannot be the cause of their own existence, it follows that something else must be their cause. If possible being cannot account for the existence of possible being, then the cause of possible being must be necessary. This is because there are only two possibilities concerning a beings existence. A being may be either possible or necessary. The following is a summary of the logic:

1. Either possible being or necessary being is the cause of possible beings.
2. Possible being is not the cause of possible beings.
3. Therefore, necessary being is the cause of possible beings.

CONCLUSION

A necessary being caused the existence of all possible beings. All the beings in our universe are possible beings. So it cannot be the case that this necessary being is identical with the universe. A necessary being must be beyond the universe. Nor could a necessary being be material, since matter is changeable and therefore possible being. It is impossible that necessary being be possible being. Therefore a necessary being cannot be material.
A necessary being could not be caused being. Only possible beings require a cause. To be necessary refers to a kind of existence that cannot not exist. That which cannot not exist does not require something to bring it into existence. Rather it brings all other things into existence. When all of these conclusions are brought together, it sounds remarkably like the God of the Christian tradition.