The Five Ways of Thomas Aquinas

Last updated:  January 28, 2010

I recently had the pleasure of reading Aquinas:  A Beginner’s Guide by philosopher Edward Feser.  Of particular relevance to this blog is the chapter on natural theology.  In order to help myself understand Aquinas’ arguments better, I wrote this summary.  I summarize his five arguments and, at the end, go over what they tell us about God.

NOTE:  This is a very rough summary and may not be entirely clear to others

The First Way

The First Way is known as the proof from motion.  It can be summarized as follows:

1. Some things are in motion.  By “motion,” Aquinas is not referring solely to objects moving from one location to another location.  He is referring to change of any kind.  We experience and observe motion, or change, every day.
2. Motion is the reduction of something from potentiality to actuality.  Actuality refers to the present state of a thing.  Potentiality refers to the potential state of a thing.  For example, suppose we have a red rubber ball.  In actuality (“in act”) this ball is solid, red, round, and bouncy.  This ball is potentially (“in potency”) blue (if we paint it), gooey (if we melt it), and so on.  Note that potentialities are rooted in a thing’s nature as it actually exists and do not include just anything.  The ball does not have the potential to, for example, fly to the moon under its own power.
3. Nothing can be reduced from potentiality to actuality except by something in a state of actuality.  For example, the ball will not go from red to blue unless blue paint actually exists.
4. Nothing can be both potential and actual in the same respect and at the same time.  For example, the ball is not potentially blue and actually blue in the same respect and at the same time.
5. It is impossible for anything to be at the same time and in the same respect both that which is moved and that which does the moving.  Whatever is in motion must be put in motion by another.  If that which put something else in motion is itself moving, there must be something further moving it, and so on.  One may object and state that animals move themselves.  In a sense they do, but animals move because their legs move, and their legs move because certain muscles flex, and certain muscles flex because of the firing of certain motor neurons, and so forth.
6. If there is an infinite series of movers there is no first mover; and if there is no first mover then there would be no other movers because the subsequent movers move only inasmuch as they are put in motion by the first mover.  Aquinas is not picturing a series of movers going back through time.  He is picturing a series of movers in the here and now.  For Aquinas, the immediate cause of an effect is simultaneous with that effect.  As an example, consider a hand moving a staff, which is moving a stone, which is moving a leaf.  This causal series is ordered “per se” or “essentially.”  The motion of the leaf depends on the motion of the stone, which in turn depends on the motion of the staff, which in turn depends on the motion of the hand.  To put it another way, the leaf moves when the stone moves, which moves when the staff moves, which moves when the hand moves.  The causes and effects are simultaneous.  If any item in this causal series is removed the leaf will no longer move.  Strictly speaking, the hand is moving everything else.  The other items are used as instruments of the hand.  It is incoherent to claim that the leaf would move without a first mover.
7. Therefore, there is a first mover, put in motion by no other, that everyone understands to be God.  Whatever else God is supposed to be, he is supposed to be the ultimate explanation of why things happen in the world.  Nothing in Aquinas’ argument implies that God is moving or is moved by something else.  Therefore it makes no sense to ask, “What moves God?”  The first mover is not reduced to act or actualized by anything else.  The first mover actualizes potentialities without itself being actualized.  This is why it is called the Unmoved Mover.  It makes other things move without itself undergoing motion.

The Second Way

The Second Way is known as the proof from causality.  It can be summarized as follows:

1. There is an order of efficient causes.  An efficient cause is that which actualizes a potency and brings something into being.  For example, the workers and machines in a factory are efficient causes in bringing a product into being.
2. Nothing can be the cause of itself for if it were it would be prior to itself, which is impossible.  In other words, that which comes into being must have an efficient cause.  Aquinas is not saying that everything has a cause, only those things that come into being have a cause.
3. In a series of efficient causes, the first cause is the cause of the intermediate causes and the intermediate causes are the cause of the ultimate cause.  Therefore, if there was no first cause there would be no intermediate causes and no ultimate cause.  As in point 6 of the First Way, Aquinas is not picturing a series of efficient causes going back through time, rather he is picturing efficient causes in the here and now.  It seems that in the First Way he is explaining why things change (in the present), while in the Second Way he is explaining why things exist (in the present).
4. Since efficient causes obviously exist, there must be a first efficient cause, which is called God.  God is a thing whose essence and existence are identical.  An essence (sometimes called nature, quiddity, or form) of a thing is that which makes it the sort of thing it is.  It is through its essence that an object can be understood.  For example, to grasp triangularity is to grasp the essence of triangles (that which makes them triangles).  Something whose essence is existence would depend on nothing else for its being (e.g., matter), since it would just be existence or being.  There could be only one such being because it would be impossible to distinguish more than one.  You could not coherently appeal to some unique form that one such thing has to distinguish it from others of its kind, for then it is not simply the act of existing but, rather, the act of existing plus something else.

The Third Way

The Third Way is known as the proof from the contingency of the world.  It can be summarized as follows:

1. There are things that either exist or do not exist (contingent things).  We observe things coming into existence and going out of existence.  Aquinas is referring to things that have a tendency to stop existing or are inherently transitory or impermanent.
2. If it is possible for something not to exist (i.e., it is contingent), there will be a point in time where it does not exist.  This simply follows from the definition in point 1.  Something that always exists clearly does not have an inherent tendency towards non-existence.  Working with this definition, it does not make sense to believe that there is a thing which could possibly go out of existence, but that does not in fact go out of existence.  Such a thing would simply not be a contingent thing under this definition.
3. If everything is contingent, then there was a point in time where nothing existed.  Given an infinite past this is true.  One might suggest that time has a beginning, but Aquinas would respond pointing out that this beginning would have to have been caused by something non-contingent.
4. That which does not exist only begins to exist by something already existing.  This ties into Aquinas’ view of causality mentioned in the summary of the Second Way.
5. If there was a time when nothing existed, there would have been no way for anything new to come into existence.
6. Since contingent things do exist, there must exist something the existence of which is necessary.
7. Every necessary thing has its necessity caused by another or not.  Those things which have their necessity caused by another are things whose essence and existence is not identical (see point 4 of the Second Way).
8. As in the case of efficient causes (in the Second Way), an infinite series of necessary causes is impossible.
9. Therefore, there must be something, which we call God, having of itself its own necessity and that causes in others their necessity (if other necessary things exist).  The only thing that could stop this infinite regress is a being whose essence and existence are identical.  The Second Way and the Third Way converge on the same conclusion but from different starting points.

The Fourth Way

The Fourth Way is known as the proof from the grades of perfection.  This is the most difficult argument for modern men to understand.  It can be summarized as follows:

1. A transcendental is something common to all beings and thus not restricted to any category or individual.  The transcendentals are:  being, thing, one, something, true, and good.  The transcendentals differ in sense but not in reference, which means that they are “convertible” with each other, in the sense that each designates one and the same thing.  As an analogy, “Superman” and “Clark Kent” refer to the same person but under different names.  It is relatively clear how being, thing, one, and something can refer to the same thing.  A thing is just a being of one kind or another.  Something refers to a being among other beings or to being as opposed to non-being.  One refers to one being distinct from others.  How is true convertible with being?  A thing is true to the extent that it conforms to the ideal defined by its essence.  For example, a sloppily drawn triangle is not as true as a carefully drawn triangle because it less perfectly instantiates the essence of triangularity.  Something “has being as the kind of thing it is precisely to the extent that it is a true instance of that kind, as defined by the universal essence existing in the intellect; and in that sense being is convertible with truth” (p. 34).  How is good convertible with being?  Aquinas thinks a thing is good to the extent that it conforms to its essence.  For example, the sloppily drawn triangle is a bad triangle while the carefully drawn triangle is a good triangle.  This sense of good is broader than the moral sense of good.
2. There are some things that are better (“more good”) than other things.
3. Things are better than other things to the extent that they more closely resemble their essence.
4. There must exist something which is truest and, consequently, something that is uttermost being.  The conclusion to the Third Way seems to entail this.
5. The maximum of any genus (category) is the cause of everything in that genus.  Recall from the Third Way that a thing which does not exist can only come into existence from something that already exists.  A being whose essence is existence is the cause of everything that has being.
6. There must be something which is the cause of the being and goodness (the transcendentals) of all beings, and this we call God.

The Fifth Way

The Fifth Way is known as the proof from finality.  It can be understood as follows:

1. Some things which lack intelligence act for an end (or multiple ends).  Aquinas believes causal regularities require explanations.  The fact that A regularly brings about B, as B’s efficient cause (recall the Second Way), entails that bringing about B is in turn the final cause of A.  This final cause of A is the end (or an end) of A.  If A was not directed towards the generation of B as its natural end we would have no way to account for the fact that A typically brings about B (rather than C, D, E, etc.).
2. Such things act designedly and not fortuitously.  Aquinas is saying that it is impossible for every apparent causal regularity to be the product of chance, for chance itself presupposes causal regularity.  Final causes are built into nature.
3. Whatever lacks intelligence can only act for an end if it is directed by something that has intelligence.  For a cause, to have any efficacy, must in some sense exist; and if it doesn’t exist in reality, then the only place for it to exist is in the intellect (p. 117).  The final causes of the universe must be in an intellect outside the universe which directs things towards their ends.  “If the world has a point, if it is for anything, this cannot be a case of unconscious teleology, since unconscious teleology is always relative to a system, is always a case of a part being for the sake of the whole. There is, by definition, no greater whole of which the world forms a part. Even if there were, we could just perform the “we cannot go on to infinity in this line” move, and quickly come to the limit. There is therefore going to be something, viz. the world, which has to have a point if anything within the world is to have a point. Apparently there are things within the world that have a point: indeed, apparently the whole structure of the world depends on things displaying tendencies and thus having points, being for something. Therefore the world as a whole does have a point, does display teleology. But this teleology cannot be unconscious, since there is no greater whole for the world to have as its unconscious point. Therefore the teleology of the world must be conscious: the point of the world must be conferred on it by some mind.”
4. An intelligent being, which we call God, exists by whom all natural things are directed toward their end(s).  Note that, again, Aquinas is concerned with the here and now and not merely the beginning of the universe (the ends are in the intellect).  This intelligent being must be being itself, otherwise it would have a potency, which like all potencies, is directed towards its end.

The Divine Attributes

The Five Ways all conclude that a being who is being itself must exist and that there can be only one such being.  Since there can only be one such being all the arguments must converge on one and the same being.  Polytheism, deism, and atheism are ruled out by Aquinas.  The one being can appropriately be called God because it has at least the following attributes:

1. As the Unmoved Mover it is immutable.
2. As Pure Act it is immaterial or incorporeal.
3. Since time cannot exist apart from change and this being does not change, it is eternal or outside time.
4. As the cause of the world it has power.
5. The Fifth Way establishes that God has an intellect.
6. Since something is perfect to the degree that it is pure act this being can be called perfect.  And since being is convertible with good we can call this being good.
7. It is simple in that it is not composed of parts.