Aquinas’ Five Ways: Text (from Summa Theologica) and Notes.
(This version of the text is from a public-domain translation by the Fathers of the English
The existence of God can be proved in five ways.
The first and more manifest way is the argument from motion. It is certain, and evident to our
senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another,
for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a
thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from
potentiality to actuality. But 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. Now 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. 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. 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; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at
a first mover, put in motion by no other; and this everyone understands to be God.
The second way is from the nature of the efficient cause. In the world of sense we find there is an
order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to
be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient
causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the
cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the
intermediate cause be several, or only one. Now to take away the cause is to take away the effect.
Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate
cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause,
neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false.
Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
The third way is taken from possibility and necessity, and runs thus. We find in nature things that
are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they
are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not
to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have
been nothing in existence. Now if this were true, even now there would be nothing in existence, because
that which does not exist only begins to exist by something already existing. Therefore, if at one time
nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even
now nothing would be in existence---which is absurd. Therefore, not all beings are merely possible, but
there must exist something the existence of which is necessary. But every necessary thing either has its
necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have
their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we
cannot but postulate the existence of some being having of itself its own necessity, and not receiving it
from another, but rather causing in others their necessity. This all men speak of as God.
The fourth way is taken from the gradation to be found in things. Among beings there are some
more and some less good, true, noble and the like. But "more" and "less" are predicated of different things,
according as they resemble in their different ways something which is the maximum, as a thing is said to be
hotter according as it more nearly resembles that which is hottest; so that there is something which is truest,
something best, something noblest and, consequently, something which is uttermost being; for those things
that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus
is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore
there must also be something which is to all beings the cause of their being, goodness, and every other
perfection; and this we call God.
The fifth way is taken from the governance of the world. We see that things which lack
intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly
always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but
designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless
it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by
the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and
this being we call God.
Notes on The Arguments:
The first four are all forms of cosmological argument. Cosmological arguments draw on features
of the world that we now observe, together with principles regarding how such features arise, to
argue that there must have been something at the very beginning which somehow initiated the
processes leading to what we now observe. This something is then identified with God. Weak
points for these arguments usually lie in the principles and in the identification step, where the
initiator of these processes is given the label, ‘God’. This weakness is less of a problem in the
fourth, where God is presented as something like the maximally excellent being, and the source of
excellence in everything else, and in the fifth, God is taken to be the designer and guide of the
world, a far more important and active role than the thin metaphysical ones assigned to God by the
first three. So it may be that, taken altogether, the God of these arguments is recognizably the
monotheistic God. But this recognition brings us back to the main question: Do these arguments
really work? I’m inclined to say not. But that doesn’t mean working through them (and seeing if
there might be a way to shore them up) isn’t worthwhile.
The Argument from Motion begins with an observation:
Some things are in motion.
It adds some principles to this, which license the following conclusion:
Everything in motion has been moved by some previous, distinct mover.
This derives from some complicated ideas about the nature of motion. Things that can move have
the potential to move; when something can move but is not actually moving, Aquinas says it is ‘in
potentiality’ with respect to motion. But when something really is moving, Aquinas says it is ‘in
act’ with respect to moving, that is, it is actually fulfilling its potential to move. But, says Aquinas
the transition from potentiality to actuality can only be brought about by something that is already
in act with respect to the quality. And nothing can be simultaneously in potentiality and in act
with respect to some quality. So nothing can move itself, i.e. actualize its own potential to move.
(Note that he ignores the possibility that something, without any cause whatsoever, might
spontaneously go from potentiality to actuality; this is of particular interest because such
spontaneous transitions are part and parcel of the standard reading of quantum mechanics.)
Finally, Aquinas rejects an infinite regress of movers:
Now we must stop somewhere.
This seems to be argued for, but the argument is pretty insubstantial. Aquinas says that without a
first mover, there would be no other movers, since the subsequent movers move only because of
the motion caused by the first mover. But the principles we have invoked so far only imply that
each mover moves (i.e. actualizes its potential to move) because of the motion of a prior mover,
which triggers its transition from potentiality to actuality. And that means only that for each
mover there must be a prior mover. Concluding from this that there must be a first mover begs the
question-- it dismisses the possibility of an infinite string of movers and moveds without offering
any principle that rules it out.
Aquinas’ conclusion, of course, is that there must be some first mover, the ‘unmoved mover’ that
started a chain of motions leading to the presently observed motions. And Aquinas blithely
declares that this first mover we all understand to be God.
This is a big jump, characteristic of the cosmological argument. It is either a cheat (if Aquinas
means us to conclude that this argument shows the existence of a being with all the properties we
usually attribute to God) or (at least) a major gap (if Aquinas holds that there is some form of
argument showing that the first mover must also have the other properties—being a person, being
benevolent, being omnipotent, etc.—that Aquinas took God to have). It would be nice if Aquinas
would at least point towards the further arguments needed to flesh out this skeletal God.
II. Efficient Cause
This argument is a cosmological argument strongly parallel to the argument from motion. But it
focuses on the existence of things and the causes of that existence, rather than on a particular
characteristic of some things (that they are moving) and the causes that bring them to actually
exhibit that characteristic.
The argument begins with an observation:
Certain things exist.
The principles at work here lead us to two conclusions. First:
Nothing causes its own existence (just as nothing causes its own motion).
Aquinas says that to cause its own existence a thing would have to be ‘prior to itself’, which is
absurd. So for each thing that exists, there must be a prior existent that caused that thing to exist.
There can be no infinite regress of existents.
Here the ‘argument’ is the same as in the argument from motion above: Each subsequent existent
exists only because of the first existent, so without a first existent nothing would exist. But this
begs the question, just as it did above. The principle we have says that each existent exists only
because of the prior existent that caused it to exist. But, unless we just assume that there must be
a first existent (thus begging the question), this does not imply that all existents depend on a first
existent. It only implies that each existent depends on some prior existent, which doesn’t rule out
an infinite sequence of existents. Further (and this point goes for the argument from motion as
well) there is no reason to suppose here that there must be a unique first existent for all that now
exists: this conclusion requires an additional assumption of common ancestry. Perhaps there are
many ‘first things’, which each began separate chains of causation leading to the things that now
The conclusion is that there must a first existent, and Aquinas again declares that we all
understand this first existent to be God. Once again, this is either a cheat, or it points towards a
further argument linking the property of being the first existent to the other properties we associate
III. Possibility and Necessity
This is another cosmological argument, but it works in a somewhat different way from the two
There are many things that exist but are capable of not existing.
The principles are, first:
That for anything that is capable of not existing, there is some time at which it doesn’t
This principle comes from Aristotle, who equated possibility in general with truth at some time.
So if it’ s possible that x does not exist, ‘x does not exist’ is true at some time, i.e. there is some
time at which x does not exist.
If everything is capable of non-existence, then at some point in time nothing exists.
(Later in the argument Aquinas assumes that this must in fact have held at some time in the past,
not just at some time in either the past or the future.) This principle is ambiguous, though. There
are two senses in which we might take it. The first treats ‘everything’ as referring to the different
individual things which exist but are capable of not existing. But then it doesn’t follow from the
non-existence of each individual thing at some ti me (even if we just assume this was some time in
the past) that there must be a time at which all individual things fail to exist. This is the obvious
reading given the context, where the focus is clearly on the different individual things that now
exist but are capable of not existing. But it undermines the argument, since we now need some
explanation of why it follows from the possibility of the individual non-existence of each thing
that exists that there must be (and in the past, too!) a time at which nothing at all exists. The
second reading takes ‘everything’ as everything taken altogether (what we might call the
mereological sum, i.e. the thing of which all these contingent things are the parts). If this, too, is
contingent, i.e. capable of non-existence (because all of its parts are), then at some point in time,
this ‘everything’ doesn’t exist, i.e. nothing exists. Read this way, the principle seems to work, but
the argument is still in trouble. After all, to say that each individual thing is capable of non-
existence is not the same as saying that everything altogether is capable of non-existence. Each
individual thing might well not exist at some time while it remains (in some sense) necessary that
something exist at every time (after all, a time where nothing exists is a time where nothing
happens—and without events and change, it’s not clear that time would continue at all). In this
case, the threat that there will turn out to be a time (in the past) when nothing at all existed doesn’t
The third principle here is:
Nothing comes from nothing (ex nihilo, nihil fit).
From this Aquinas concludes that if everything is capable of non-existence, there would be
nothing at all now, since at some time in the past there would have been nothing, and from then on
there could be nothing at all, ever. Everything that exists but at some (prior) time did not exist
was brought into existence by something prior, i.e. something that already existed at the time it
was brought into existence. Once we got down to nothing at all existing, we’d be stuck there
forever. (See the argument from efficient causation, above.)
The upshot is that there must be some being that exists necessarily, and therefore exists at every
time, to ensure that there is always something that could bring the other, contingent beings into
Finally, at the end of the third way, Aquinas mounts another regress argument like the first two,
this time regarding the source of the necessary being’s necessity. It must either come from
another necessary being, or from within its own nature. But, since this cannot go to infinity (the
argument for this parallels the weak ones against infinite sequences offered in the first two ways),
Aquinas concludes there must be a necessary being that is the source of its own necessity (and of
the necessity of all other necessary beings). Again, Aquinas moves immediately to identify this
being as God. As with the first two arguments, this one also gives (and promises) no explicit
account of the connections between this characteristic of God and the other properties we
generally attribute to her.
This is the last of the cosmological arguments, and it treads very close to the teleological, since the
‘gradation’ spoken of here is a gradation of some kind of ‘excellence’; the principles invoked here
have to do with the natural workings of the world, but they are also applied to values (which
makes the conclusion of this argument more satisfying than that of the previous 3).
The observation, of course, is just that there are things with varying degrees of excellence (i.e. that
are more or less ‘good, true, noble, and the like’).
The principles emerge from an account of such gradations:
Gradation only makes sense, says Aquinas, in terms of greater or lesser similarity to a thing that is
maximally good, true, or noble.
God, of course, is this maximal thing. Here Aquinas does some of the metaphysical linking that
was missing from the first three arguments (though I don’t see an argument that maximal
goodness, truth and nobility must all be embodied in the same thing):
Aquinas urges, first, that there is a connection between maximality in these virtues and ‘uttermost
(i.e. maximal) being’. Further, Aquinas says that in general the maximally thing is the cause of
the ness in all things (as fire, the maximally hot thing, is the cause of heat in everything else
that is hot); thus God is the cause of whatever goodness, truth, nobility and existence there is in
everything else. On the whole, I think the picture of God this argument leads to is by far the
closest to being reasonably complete. But there are a lot of doubts about this ‘maximality’
principle and especially the causal condition requiring that the maximally being be the cause of
whatever ness exists in everything else.
This last of the five ways begins with a much more complicated kind of ‘observation’:
Things which ‘lack knowledge’ nevertheless act, always or nearly always, in ways that lead to ‘the
Aquinas concludes from this that these things act ‘for an end’ (i.e. in order to achieve some goal).
From this, because they themselves lack knowledge, Aquinas concludes that they act ‘designedly’,
i.e. by the design or intent of an intelligent being, since otherwise they could not be expected to
regularly give rise to ‘the best’.
The key principle, this time, is that if a thing that lacks knowledge nevertheless regularly manages
to achieve some end, it must be directed towards that end by something that possesses knowledge.
So, themselves lacking knowledge, these things do not aim at the end or have designs by
themselves. Instead, they are directed towards their goals (as the archer aims an arrow at the
target) by some intelligent being.
Aquinas rushes to the conclusion that there is one being who directs all natural objects towards
their ends, whom we call God. As with I-IV above, there is nothing to show that all these
‘designs’ derive from a single designer. (One might even argue that the conflicts we see in the
natural world suggest a multiplicity of competing designers._ More importantly, there is no
general argument showing that no process involving only natural things that lack knowledge could
possibly give rise to the apparent purposiveness of things (especially living things) in the natural
world. In particular, where that purpose involves survival and reproduction, natural selection
provides a credible alternative. Needless to say, Aquinas is blameless for passing this particular
alternative by—it wasn’t proposed until 1842s, when Darwin first sketched his ideas on species in
some detail. But it was still jumping to conclusions to assume that no natural account of this
apparent ‘purpose’ is possible.
An important response to this kind of argument appears in Hume’s Dialogues Concerning Natural
Religion Hume points out that if the principle, that things that lack knowledge cannot regularly
achieve any goal or purpose unless directed by some being that possesses knowledge, is supposed
to be something we have observed to be true, there is a problem. After all, we have often observed
that animals and plants are able to nourish and reproduce themselves, without any apparent
intervention or guidance from an intelligent being. So it is only by assuming some such intelligent
guide that we can prevent this principle from being obviously refuted at the outset. As a result,
Hume argued, the argument from design is actually circular—we have to assume its conclusion
from the beginning in order to defend one of its premises!
The very short passage here from Clarke (Newton’s defender in a bitter exchange of
correspondence with Leibniz) expands on some of the issues central to Aquinas’ cosmological
arguments, especially the second way. The most important element here is the vigourous effort to
make a case against the notion of an infinite sequence of contingent beings. Note that Clarke
treats the entire resulting sequence as a single thing, seizing on the mereological reading of
Aquinas’ ambiguous premise in the third way. Does this mereological assemblage need a separate
cause? Clarke argues that it, too, is a contingent thing and so does need a cause outside iteelf.
(But by Aristotle’s criterion, it’s a necessary thing, so Clarke has to reject Aristotle’s link between
contingency and non-existence at some time. But then it seems that a contingent thing could
nevertheless exist forever, and we need an argument for the claim that it would still need a cause
of its existence.)
Clarke’s argument here seeks reasons for the existence of things—and if these reasons are not to
be found in the thing itself, then he claims it must depend on some other thing for its existence.
So we have a link here to the point made in class on Tuesday: Clarke, like Aquinas, is ruling out
the possibility that things just are (and just come to be) by chance and happenstance. There must
be a ‘sufficient reason’ for whatever exists, and this requirement applies not just to individual
things, but to the whole sequence of individual things as well.
Note, however, the far greater modesty of Clarke’s conclusion, where he explicitly acknowledges
the need to do more to settle just what this immutable (unchangeable) and independent being he
has argued for, really is.
Rowe on the Cosmological Argument.