God’s existence can be known by the light of natural reason - LifeSite

OPINION
God’s existence can be known by
the light of natural reason
The 'five ways' of St. Thomas Aquinas are not the only ways to demonstrate God’s
existence. But they are sufficient to show that God certainly exists, and that
man’s reason, unaided by the light of divine revelation, can arrive at the certain
knowledge of this fact.
Matthew McCusker
Thu Feb 8, 2024 - 1:32 pm EST
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The following is Part IV in a series defending the claims of the Catholic Church. Read Part I
here; Part II here; and Part III here.
(LifeSiteNews) — In the previous installments of this series, we have seen that:
1. The existence of God is not known to us immediately and intuitively, but
2. The existence of God is something that is open to demonstration.
It is now time to demonstrate the existence of God by philosophical argument.
St. Thomas Aquinas summarized “five ways” to carry out this demonstration in his Summa
Theologica.
Each of these “five ways” is a compelling proof, which begins with consideration of a truth
that can be known with certainty from observation of the world around us. Each of the “five
ways” seeks to demonstrate the existence of God – “the cause” – from the study of material
creation – “the effect.” Thus, they are a posteriori arguments of the kind we discussed in the
previous article.
The “five ways” are not the only ways to demonstrate God’s existence. But they are sufficient
to show that God certainly exists, and that man’s reason, unaided by the light of divine
revelation, can arrive at the certain knowledge of this fact.
Sometimes the simplicity and power of the five ways is missed by modern readers because of
a lack of familiarity with the terminology used.
In this article I will explain the “first way” in language which is familiar to all of us.
The argument from motion (or change)
The first of the five ways is the argument from motion. This proof was first outlined by the
ancient Greek philosopher Aristotle. St. Thomas Aquinas regarded this as the “most manifest
way” – that is, the most obvious – and he discussed it a number of times in different works.
The word “motion” is a stumbling block for many people.
Modern readers may assume that the word “motion” means only the movement of an object
through space and time, i.e. the motion of a comet through the sky, a car along the road, or a
leaf falling from a tree. However, the meaning of the word as used by St. Thomas is broader
than that.
The meaning of the word “motion,” as it is used in the “first way,” is better reflected by our
word “change” – the movement of a thing from one state to another – and so I will use the
word “change” interchangeably with “motion” throughout the rest of this article.
The things we experience are subject to change
Everyone can observe that things in the world around us are subject to change. As St.
Thomas states:
It is certain, and evident to our senses, that some things are in motion.[1]
Water, left outside in the sun, will evaporate, or if left in a sufficiently cold temperature, will
freeze. Wood, left in a damp environment will rot, or dry wood, encountering fire, will burn.
Fruit grows, ripens and rots, and from its seeds new trees grow. Livings things come into
existence, grow, and then die.
No human being can deny the existence of change. It is a reality that we all recognize.
Things change in some ways but not in others
Another reality that we all recognize is that when a thing changes, it can only change in a
limited number of ways.
Water may freeze, or evaporate, but it doesn’t become oil, or in the absence of an additive,
become purple or become toxic. Similarly, a piece of wood may burn, or rot, but it will not
turn into cheese or ice cream or begin to speak.
Thus, we see that everything which exists has a number of potential ways in which it can be
in motion (change) but that the range of these potentialities is limited by the nature of the
thing.
Act and potency
From the discussion above we can now recognize that at any given moment a thing can be
considered: (i) as it is now and (ii) as it might become.
When we speak about how a thing actually is at any given moment, we are speaking about
how it is “in act.”
And when we consider how a thing might potentially change, we are considering its
“potency.”
Therefore, we can conclude that all the things of which we have sensory knowledge are a
composite of act and potency, of what they are now and of what they might become.
When the potentiality of a thing is actualized, we say it has been “reduced to act.” The use of
the word “reduced” indicates that whereas the potentialities of a thing are many, it is
actualized as only one of them at that moment in time.
Thus, when water is boiled or wood is burned a particular potential change has taken place,
to the exclusion of others. If water is boiling, it cannot also be freezing; if wood is burnt to
ash, it cannot also be rotten.
An agent needed to reduce potency to act
Now that we have recognized that all the things of which we have sense knowledge are a
composite of act and potency, we are ready to notice something else.
We will notice that whenever something changes, it is as a result of something other than
itself. For example, the water only boils because an agent, such as a fire or a gas stove, has
imparted heat to it. Similarly, if water freezes, it is because something has changed in the
environmental conditions which has reduced the temperature. Water will not boil in the
absence of heat, nor will it freeze when sufficient heat is present. Therefore, we can say, with
St. Thomas:
Nothing can be reduced from potentiality to actuality, except by something in a state
of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to
be actually hot, and thereby moves and changes it.
Furthermore, a thing which is potentially in one state, is actually in a different state. It cannot
be potentially and actually the same thing at the same time. For example, water which is at
10°C cannot be at 100°C in the same moment. Wood cannot be both “on fire” and “not on fire”
simultaneously. As St. Thomas says:
It is not possible that the same thing should be at once in actuality and potentiality in
the same respect, but only in different respects. For what is actually hot cannot
simultaneously be potentially hot; but it is simultaneously potentially cold.
St. Thomas concludes:
It is therefore impossible that in the same respect and in the same way a thing should
be both mover and moved, i.e. that it should move itself. Therefore, whatever is in
motion must be put in motion by another.
It is impossible for water to boil itself because non-boiling water lacks sufficient heat energy
to boil. If it had that energy, it would already be boiling. A heat source is required to put the
water molecules in motion, such that the water begins to boil. And this heat source must be
capable of providing sufficient heat to raise the temperature to boiling point. That is, a cause
must be proportionate to its effect.
A common objection to the assertion that “whatever is in motion must be put in motion by
another” is that living things move themselves.
However, on closer examination, we will see that this objection is easily overcome.
Consider a cat chasing a mouse. The legs of the cat do not run of their own accord. Their
potential to run is actualized by the cat’s nervous system. That specific potential nervous
system response was actualized by the brain. The potential of brain was actualized by the
sense of sight or smell, which detected the presence of the mouse. We could continue the
sequence of changes back from that point, through the life of the mouse and the cat, right
back to their first existence, when the potential of mouse and cat reproductive cells was
made actual in the procreation of a mouse and a cat. From there we could continue back
through the sequence of potency reduced to act, to the first cat and the first mouse, and to the
first mover of the first cat and the first mouse.
Perform the thought experiment yourself: can you think of anything that exists, in which
change originates in the specific thing that changes, without the change being the result of
some other agent?
If you reflect on this, I think you will reach the same conclusion as St. Thomas:
Whatever is in motion must be put in motion by another.
How does this prove the existence of God?
So far, we have seen that:
1. The things of which we have sensory awareness are subject to change (motion)
2. These changeable things are all composites of act and potency.
3. A thing is reduced from potency to act, by something other than of itself.
This presents a problem which requires a solution.
If everything is reduced from potency to act by something other than itself, then nothing can
act on anything else unless it has first been acted on by another.
But if everything must be acted on by another before it can act itself, how was anything first
put into act?
We would be faced with a universe of composite beings which would never have the power
to effect motion in other things unless they had first been put in motion by one of those other
things.
An endless sequence of things reducing other things to act is not possible, unless there is a
cause which is, of itself, able to reduce the sequence to act.
As St. Thomas wrote:
If that by which it is put in motion be itself put in motion, then this also must needs be
put in motion by another, and that by another again.
But this cannot go on to infinity, because then there would be no first mover, and,
consequently, no other mover; seeing that subsequent movers move only inasmuch as
they are put in motion by the first mover.
Therefore, we can see that it is absolutely impossible for a universe to exist which consists
only of beings that are a composite of act and potency.
But given that we know that the composite beings that we experience with our senses do
exist, there must be a solution to the problem.
Pure Act
To account for the universe that we observe with our senses, we must posit the existence of a
being which does not have the limitation inherent in composites of act and potency.
In fact, we must posit something very specific. We must posit a being which:
1. reduces other beings to act
2. does not need to be reduced to act itself.
That is, we must posit a being, which, in and of itself, is act without potency, a being which,
by its very nature, is fully actualized, without any potency to be reduced.
Only such a being, which philosophers call “Pure Act,” resolves the problem outlined above.
Such a being must exist, in order for the universe that we observe to exist. Such a cause must
exist, in order for the effects that we observe to exist.
As St. Thomas Aquinas concludes:
[I]t is necessary to arrive at a first mover, put in motion by no other; and this everyone
understands to be God.
The being that is the First Mover is the cause of all the motion, of all the change, that we
observe in the universe. The being that is Pure Act has no potency because it is necessarily
fully actualized. It must therefore be eternal and unchanging. This is the being “with whom
there is no change, nor shadow of alteration.” (James 1:17)
This truly is the being that is signified by the word “God.”
References
1
All quotes are taken from St Thomas’s account of the first way, as found in ST. I. q2. a3.
TOPICS
Catholic Church
Commentary
Faith
TAGGED AS
a posteriori
demonstration
act and potency
Existence of God
Philosophical Argument
Summa Theologica
actuality
argument from motion
causality
Change
first mover
five ways
James 1:17
Natural Theology
philosophy of religion
potentiality
Pure Act
St. Thomas Aquinas
Theology
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