Five Ways (Aquinas)

The Quinque viæ are five logical arguments for the existence of God summarized by the 13th-century Catholic philosopher and theologian Thomas Aquinas in his book Summa Theologica. They are:

Aquinas expands the first of these – God as the "unmoved mover" – in his Summa Contra Gentiles.

Background

Need for demonstration of the existence of God

Aquinas thought the finite human mind could not know what God is directly, therefore God's existence is not self-evident to us, although it is self-evident in itself. On the other hand, he also rejected the idea that God's existence cannot be demonstrated: although it is impossible to give a so-called propter quid demonstration, going from the causes to the effects; still, the proposition God exists can be "demonstrated" from God's effects, which are more known to us, through a so-called quia demonstration. However, Aquinas did not hold that what could be demonstrated philosophically would necessarily provide any of the vital details revealed in Christ and through the church —quite the reverse. For example, while he would allow that "in all creatures there is found the trace of the Trinity", yet "a trace shows that someone has passed by but not actually who it is."

Categorization

The first three ways are generally considered to be cosmological arguments. Aquinas omitted various arguments he believed to be insufficient or unsuited, such as the ontological argument made by Anselm of Canterbury.

Essential and accidental causal chains

The first two Ways relate to causation. When Aquinas argues that a causal chain cannot be infinitely long, he does not have in mind a chain where each element is a prior event that causes the next event; in other words, he is not arguing for a first event in a sequence. Rather, his argument is that a chain of concurrent or simultaneous effects must be rooted ultimately in a cause capable of generating these effects, and hence for a cause that is first in the hierarchical sense, not the temporal sense.
Aquinas follows the distinction found in Aristotle's Physics 8.5, and developed by Simplicius, Maimonides, and Avicenna that a causal chain may be either accidental or essential
His thinking here relies on what would later be labelled "essentially ordered causal series" by John Duns Scotus.
This is also why Aquinas rejected that reason can prove the universe must have had a beginning in time; for all he knows and can demonstrate the universe could have been 'created from eternity' by the eternal God. He accepts the biblical
doctrine of creation as a truth of faith, not reason.
For a discussion of a causal chain argument that is based on a created beginning, see Kalam cosmological argument.

The Five Ways

First way: The Argument of the Unmoved Mover

Summary

In the world, we can see that at least some things are changing. Whatever is changing is being changed by something else. If that by which it is changing is itself changed, then it too is being changed by something else. But this chain cannot be infinitely long, so there must be something that changes others without itself changing. This everyone understands to be God.

Explanation

This argument is a summary of a much longer argument, which is in the Summa Contra Gentiles, which itself is a summary of a much longer argument given by Aristotle in both The Physics, and The Metaphysics. Most arguments in the Summa are shortened arguments of much longer disputations. Hence, why some translate, "Summa Theologiae," as, "Summary Of Theology."
Aquinas uses the term "motion" in his argument, but by this he understands any kind of "change", more specifically a transit from potentiality to actuality. Since a potential does not yet exist, it cannot cause itself to exist and can therefore only be brought into existence by something already existing.
Suarez contested the Aristotelian principle according to which all that moves is moved by something else, noting that living beings are capable of moving by themselves and are not moved by anything else, and that the heavens could be moved by a form internal to them. He then reformulated the principle to omne quod fit ab alio fit , and created the following argument:

Second way: The Argument of the First Cause

Summary

In the world, we can see that some things are caused. But it is not possible for something to be the cause of itself because this would entail that it exists prior to itself, which is a contradiction. If that by which it is caused is itself caused, then it too must have a cause. But this cannot be an infinitely long chain, so, there must be a cause which is not itself caused by anything further. This everyone understands to be God.

Explanation

As in the First Way, the causes Aquinas has in mind are not sequential events, but rather simultaneously existing dependency relationships: Aristotle's efficient cause. For example, plant growth depends on sunlight and water, which depend on "ideal atmospheric activities", which are "governed by more fundamental causes", and so on. Aquinas is not arguing for a cause that is first in a sequence, but rather first in a hierarchy: a principal cause, rather than a derivative cause.

Third way: The Argument from Time and Contingency

Summary

In the world we see things that are possible to be and possible not to be. In other words, perishable things. But if everything were contingent and thus capable of going out of existence, then, eventually everything would perish, and nothing would exist now. But things clearly do exist now. Therefore, there must be things that are imperishable: necessary beings. But some necessary beings receive their incorruptibility from something else. Those necessary beings which give the power of incorruptibility may also receive their incorruptibility, and those who give them their power may also receive their incorruptibility and so on. But this cannot go on forever, and so we come to a necessary being that has not received their necessity, and is necessary in itself. And this everyone understands to be God.

Explanation

This argument seems to be a summary of another longer argument called, "Proof Of The Truthful," given by the Islamic theologian, Avicenna, who argued for the existence of a necessary being like the first half of this argument. But, Aquinas didn't think that finding a necessary being was enough, especially since he believed that human souls, and angels were incorruptible necessary beings. And so, he added on the second half of the argument which follows like the first two ways, i.e. the impossibility of an infinite regress in causation.
The argument begins with the observation that things around us come into and go out of existence: animals die, buildings are destroyed, etc. But if everything were like this, then, at some time nothing would exist. Some interpreters read Aquinas to mean that assuming an infinite past, all possibilities would be realized and everything would go out of existence. Since this is clearly not the case, then there must be at least one thing that does not have the possibility of going out of existence. However, this explanation seems to involve the fallacy of composition. Moreover, it does not seem to be in keeping with Aquinas's principle that, among natural things, the corruption of one thing is always the generation of another. Alternatively, one could read Aquinas to be arguing as follows: if there is eternal change, so that things are eternally being generated and corrupted, and since an eternal effect requires an eternal cause, then there must exist an eternal agent which can account for the eternity of generation and corruption. To hold the alternative, namely that an infinite series of contingent causes would be able to explain eternal generation and corruption, would posit a circular argument: Why is there eternal generation and corruption? Because there is an eternal series of causes which are being generated and corrupted. And why is there an infinite series of causes which are being generated and corrupted? Because there is eternal generation and corruption. Since such an explanation is not acceptable, there must be eternal and necessary being.

Fourth way: The Argument from Degree

Summary

We see things in the world that vary in degrees of goodness, truth, nobility, etc. For example, well-drawn circles are better than poorly drawn ones, healthy animals are better than sick animals. Moreover, some substances are better than others, since living things are better than non-living things, and animals are better than plants, in testimony of which no one would choose to lose their senses for the sake of having the longevity of a tree. But judging something as being "more" or "less" implies some standard against which it is being judged. For example, in a room full of people of varying heights, at least one must be tallest. Therefore, there is something which is best and most true, and most a being, etc. Aquinas then adds the premise: what is most in a genus is the cause of all else in that genus. From this he deduces that there exists some most-good being which causes goodness in all else, and this everyone understands to be God.

Explanation

The argument is rooted in Aristotle and Plato but its developed form is found in Anselm of Canterbury's Monologion. Although the argument has Platonic influences, Aquinas was not a Platonist and did not believe in the Theory of Forms. Rather, he is arguing that things that only have partial or flawed existence indicate that they are not their own sources of existence, and so must rely on something else as the source of their existence. The argument makes use of the theory of transcendentals: properties of existence. For example, "true" presents an aspect of existence, as any existent thing will be "true" insofar as it is true that it exists. Or "one," insofar as any existent thing will be "one thing."
The premise which seems to cause the most difficulty among interpreters of the fourth way is that the greatest in a genus is the cause of all else in the genus. This premise does not seem to be universally true, and indeed, Aquinas himself thinks that this premise is not always true, but only under certain circumstances: namely, when 1) the lesser things in the genus need a cause, and 2) there is nothing outside the genus which can be the cause. When these two conditions are met, the premise that the greatest in the genus is the cause of all else in that genus holds, since nothing gives what it does not have. Since Aquinas is dealing specifically with transcendentals like being and goodness, and since there is nothing outside the transcendentals, it follows that there is nothing outside the genus which could be a cause. Moreover, if something has less than the maximum being or goodness or truth, then it must not have being or goodness or truth in itself. For example, how could what has circularity itself be less than fully circular? Therefore, whatever has less than the maximum being or goodness or truth must need a cause of their being and goodness and truth.