This translation is from the Summa Theologiae of St. Thomas Aquinas, Second and Revised Edition, translated by Fathers of
the English Dominican Province,1920. This translation is now in the public domain. The analysis of the argument which
follows is © 2004 Theodore Gracyk and is used with attribution.
It is often assumed that the Ways are meant to be self-sufficient proofs, which is something of a mistake. Thomas intends the
Ways to cogently describe, as he puts it, “what all people mean when they say ‘God.’” These Ways are how God can be
meant, when the term God is spoken of by reason alone. Thomas does not, in fact, believe that complete knowledge of God is
possible without recourse to revelation, that is, by special knowledge of God given by God and God alone. But Roman
Catholic “natural theology” following Aristotle, holds that natural human reason can know that God is, apart from revelation
(or “revealed theology”). To know what or who God is, of course, requires religious knowledge which comes from divine
revelation. So it is important to understand what the Five Ways are, and what they are not. It has also been noted that they are
meant to be taken together; no one of the arguments is particularly convincing by itself.
The Ways are taken from Aristotelian science as found in the Physics and Metaphysics, as well as the formulations of
Maimonides in the Guide to the Perplexed and ibn Sînâ’s Arabic commentaries on Aristotle (both profound influences on
Thomas’s thought). Thomas uses them for his own religious purposes, of course, but they are not strictly original to him. They
are legitimately associated with him, however, because of the profound impact his discussion of them has had on subsequent
philosophy, and also because it was Thomas that organized them into their present form and who collected them together into
I have interpolated below reference to Aristotle’s own arguments to show Thomas’s source, so that you can see the
Aristotelian origin of the Five Ways (something all too many thinkers ignore or sometimes even deny, oddly), and also so that
you can see how Thomas explicated them in his own terms.
From Summa Theologiae
Part one, Question 2, Article 3
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.
[“Substances are the primary reality, and if they are all perishable then everything is perishable. But motion cannot be either
generated or destroyed, nor time…And there is no continuous motion except that which is spatial…and circular…Therefore
the ‘ultimate heaven’ must be eternal. And since that which moves while itself moving is intermediate, there must be a first
mover that is not itself moved.” (Aristotle, Metaphysics 1071b-72a)]
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.
[Aristotle’s “first mover that is not itself moved” (prôton kinoun akinçton) is not only the source of motion, but the source of
sources; that is, along with being the unmoved mover, it is the uncaused causer. All causes require a cause (there are no causal
loops and no infinite causal chains, according to Aristotelian physics). And this, Thomas says, is what “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.
[“That which has potentiality may not actualize it. Thus it will not help if we postulate eternal substances…unless there be in
them some principle which can cause change. And even this is not enough…for unless it actually functions there will not be
motion. And it will still not be enough even if it does function, if its essence is potentiality, for there will not be eternal
motion, since that which exists potentially may not exist. Therefore there must be a principle of this kind whose essence is
actuality.” (Aristotle, Metaphysics 1071b)]
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.
[“But since there is something, X, which moves while being itself unmoved, existing actually, X cannot be otherwise in any
respect…Thus X is necessarily existent; and as necessary it is good, and in this sense is a first principle. For ‘the necessary’
has all these meanings [including] ‘that without which excellence is impossible’… Such then is the first principle upon which
depend the sensible universe and the world of nature. And its life is like the best that we can enjoy only temporarily, but it
must be in that state always.” (Aristotle, Metaphysics 1072b)]
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
[“We must also consider in which sense the nature of the universe contains the good or supreme good; whether as something
separate and independent, or as the orderly arrangement of its parts. Probably in both senses, as an army does; for the
efficiency of the army consists partly in the order and partly in the general; but chiefly in the latter…all things, both fishes and
birds and plants, are ordered together in some way…everything is ordered together in one way.” (Aristotle, Metaphysics
The First Way: Argument from Motion
- Our senses prove that some things are in motion.
- Things move when potential motion becomes actual motion.
- Only an actual motion can convert a potential motion into an actual motion.
- Nothing can be at once in both actuality and potentiality in the same respect (i.e., if both actual and potential, it is actual
in one respect and potential in another).
- Therefore nothing can move itself.
- Therefore each thing in motion is moved by something else.
- The sequence of motion cannot extend ad infinitum.
- 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: Argument from Efficient Causes
- We perceive a series of efficient causes of things in the world.
- Nothing exists prior to itself.
- Therefore nothing is the efficient cause of itself.
- If a previous efficient cause does not exist, neither does the thing that results.
- Therefore if the first thing in a series does not exist, nothing in the series exists.
- The series of efficient causes cannot extend ad infinitum into the past, for then there would be no things existing now.
- Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
The Third Way: Argument from Possibility and Necessity (Reductio argument)
- We find in nature things that are possible to be and not to be, that come into being and go out of being i.e., contingent
- Assume that every being is a contingent being.
- For each contingent being, there is a time it does not exist.
- Therefore it is impossible for these always to exist.
- Therefore there could have been a time when no things existed.
- Therefore at that time there would have been nothing to bring the currently existing contingent beings into existence.
- Therefore, nothing would be in existence now.
- We have reached an absurd result from assuming that every being is a contingent being.
- Therefore not every being is a contingent being.
- Therefore some being exists of its own necessity, and does not receive its existence from another being, but rather causes
them. This all men speak of as God.
The Fourth Way: Argument from Gradation of Being
- There is a gradation to be found in things: some are better or worse than others.
- Predications of degree require reference to the “uttermost” case (e.g., a thing is said to be hotter according as it more
nearly resembles that which is hottest).
- The maximum in any genus is the cause of all in that genus.
- 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: Argument from Design
- We see that natural bodies work towards some goal, and do not do so by chance.
- Most natural things lack knowledge.
- But as an arrow reaches its target because it is directed by an archer, what lacks intelligence achieves goals by being
directed by something intelligence.
- Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.