On Aquinas’ Five Ways
In his seminal work, Summa Theologica, the celebrated Christian philosopher, Thomas Aquinas, engages with the question of the existence of God. He notes that there are certainly objections to the claim that there exists such a divinity, but Aquinas believes that these objections can be overcome and that this existence can be shown to be well-founded. The eminent philosopher then lays out a list of arguments which he supposes to make this case. These arguments have come to be known as Aquinas’ Five Ways, and they have been so influential in philosophy that many theologians and apologists still cite them as if they are authoritative logical proofs, more than 700 years after the Italian priest set them to page. On the contrary, however, it seems that there are a number of issues which prevent Aquinas’ Five Ways from being quite so powerful, now, as they may have been in his own day.
The First Way is Aquinas’ Argument from Motion. The philosopher, being heavily entrenched in Aristotelian philosophy, means something different by “motion” than is commonly understood, today. When Aquinas says that a thing is “in motion,” he means that some change which has yet to occur is being made in that thing. In addition to spatial mobilization, this can refer to any sort of change—for example, a chameleon which is perfectly still, but changing colors, would be “in motion,” on Aquinas’ view. We can summarize this argument as follows:
- There are some things which are in motion.
- If a thing is in motion, that motion must originate in the action of some other thing.
- This chain cannot go to infinity, because there must be a First Mover.
- Therefore, there is a First Mover whose motion does not originate in the action of some other thing. (3)
Laid out in this fashion, it seems perfectly clear that Aquinas’ First Way is not even a valid argument, let alone a sound one. His third premise is a blatant instance of question begging. Aquinas asserts that there must be a First Mover, therefore there is a First Mover. Furthermore, the conclusion which he draws violates his second premise, making it a fairly clear example of special pleading. If Aquinas’ conclusion is true, then his second premise is false, invalidating the argument.
Aquinas’ dedication to Aristotle is again evident in his Second Way, the Argument from the Formality of Efficient Causation. Aristotle’s view of the notion of causation divided the concept into four categories: material, efficient, formal, and final. The “material” cause is that which is being changed, the “efficient” cause is the agent effecting the change, the “formal” cause is the manner in which the change occurs, and the “final” cause is the end purpose which drove the change (Aristotle’s Physics 2.3). Using an argument which is similar to his First Way, Aquinas argues:
- Changes have Efficient Causes.
- No change can be its own Efficient Cause.
- There cannot be an infinite chain of Efficient Causes.
- Therefore, there must be a First Efficient Cause. (1, 2, 3)
This argument is slightly better than Aquinas’ First Way, as this one is at least valid and free of overt logical fallacies. However, it is not necessarily sound. The third premise is fairly suspect, and Aquinas does not give very good support for it. The priest notes that in any series of efficient causes, a subsequent efficient cause is always the result of a prior. He then baldly asserts that there must be a first Efficient Cause. This, again, shows Aristotle’s influence on Aquinas, as the Greek philosopher was vehemently opposed to the notion of actually infinite sets of entities (Physics 6.1). Unfortunately for Aquinas, Aristotle’s understanding of infinity is nearly as antiquated as was his belief that the Sun, planets, and stars revolve around the Earth. Mathematicians and physicists have been regularly utilizing completed infinite sets in real-world applications for more than 400 years, now, and it has been nearly 100 years since the concept has even been a source of much controversy in the philosophy of mathematics. If Aquinas wants to demonstrate that there cannot be an infinitely receding chain of Efficient Causes, he must do a better job than to simply note the fact that this would imply that the chain lacks a first cause.
The Third Way is an Argument from Contingency. Aquinas points out that there are things which exist that exhibit the potential to not exist. Four centuries later, in a similar argument, Leibniz would say that such things are “contingent,” as opposed to “necessary” things—that is, things which cannot logically be non-existent. Aquinas’ Third Way is summed up as follows:
- Some things exist which have the potential to cease existing.
- If a thing can cease to exist, then there must have been a prior time in which it did not exist.
- If everything has the potential to cease existing, then there must have been a time when nothing existed. (1, 2)
- If nothing existed, nothing can have been brought into existence.
- Therefore, it cannot be that everything has the potential to cease existing, since (4) contradicts with (1).
There are two problems with this Third Way which are of particular concern. Firstly, Aquinas offers absolutely no justification for his second premise. It does not follow that a thing potentially ceasing to exist implies that it must have not existed at some time in the past. However, even if that premise is true, the most which Aquinas might draw from this argument is that there must exist at least one thing which is necessary. He has not demonstrated that there can only be a single necessary thing. Aquinas gives no reason to think that there might not be a multitude of necessary things which do not have their necessity caused, but rather exist necessarily in and of themselves. Certainly, if numerous such things were in existence, Aquinas would not say that the whole set of them is what is meant by the name, “God.”
Aquinas proposes an Argument from Gradation as his Fourth Way. He discusses the manner in which things are described, noting that some things exhibit a certain property to a greater or lesser scale than do others. He claims that this gradation is judged in comparison to some singular exhibitor which most exemplifies that property.
- There are some properties of things which can be accorded as greater or lesser.
- A property can only be accorded as greater or lesser as it resembles a thing which is the greatest exhibitor of that property.
- “Being,” “goodness,” and every other perfection can be accorded as greater or lesser in a thing.
- Therefore, there must exist a greatest exhibitor of “being,” “goodness,” and every other perfection. (1, 2, 3)
The second premise of this argument is worse than just dubious. Indeed, it would seem to be glaringly false. In his own example supporting the premise, Aquinas states that “hotter” is judged in comparison to fire, which must be considered “hottest,” since it is the source of all heat– yet another antiquated and abandoned position which Aquinas acquired from Aristotle. However, it is now known that fire is not the source of all heat, and that there exist things which are far, far hotter than fire. Even more damning to this perspective, however, is that a person need not ever know anything of fire to know that a stone which had been left out in the sun is hotter than one left in the shade. Nor is it even clear that “hottest” is a notion which can be applied to any single, universal thing. The gradation of a property is not judged by comparison to the maximal extremes of that property, but rather by comparison to other things which exhibit that property. An oven is hotter than a refrigerator and a blast furnace is hotter, still, but these are all still exceptionally cold in comparison to the surface temperature of a star.
Finally, the Fifth Way is an Argument from Teleology. Aquinas claims that there exist things which lack intelligence, and which yet act to a purposeful end. This purpose, he claims, must be instilled into those things by an agent with intelligence.
- There exist things which lack intelligence and act for some purpose.
- A thing which lacks intelligence cannot act for a purpose unless it is directed to do so by a being with intelligence.
- Therefore, some intelligent being exists by whom all things which lack intelligence are directed to their purpose. (1, 2)
Once again, Aquinas’ formulation seems problematic. Firstly, it is not clear that things which lack intelligence are acting in accordance with some distinct purpose. His attempted justification for this point does not seem to really lend it much support. Aquinas claims that these things act “always, or nearly always, in the same way, to obtain the best result.” It is far from demonstrated that this is the case; however, even if it is, it does not follow that that their actions are therefore directed by an intelligent being. Aquinas simply asserts that this is so. Again, this would seem to be a function of the priest’s reliance upon Aristotle, and his ignorance of the physics which would be discovered centuries after his Summa. Furthermore, as with the Third Way, Aquinas gives no good reason to think that all of these natural, intelligence-lacking things must therefore be directed by a single entity, other than the earlier argument regarding Efficient Causation which has already been seen to be problematic. Even if his premises were true, Aquinas is not justified in this leap of logic. The planets, for example, might each be driven in their orbits by an intelligent being distinct from that driving any of the others, and neither of the first two premises would be violated. As before, it is doubtful Aquinas would agree that all intelligent beings are what is meant by the name, “God.”
The Five Ways of Thomas Aquinas’ Summa Theologica are, to be certain, an important part of the history of philosophy. However, their brilliance and validity are very often overstated by proponents. Aquinas’ views are mired in a foundation of the positions of Aristotelian physics which, though extremely influential in 13th and 14th Century Christendom, are now extremely antiquated and which have been abandoned by all serious scholarship. Even within that framework, however, his arguments are rather loosely proffered and lacking in rigor. Aquinas’ Five Ways do not exhibit the strength of argument with which they are commonly characterized. As such, the question of God’s existence does not seem to be quite so well founded as the Italian priest claimed.