“Aquinas’ Cosmological Argument proves that God exists.” Critically evaluate this statement.  [40] 

Cosmological Arguments start with the existence of the universe (Greek = Kosmos) and conclude that God is the most logical explanation of it.  They are some of the oldest arguments for God’s existence and have an intuitive appeal.  As Richard Swinburne observed in “Is There a God?” (1996)

The human quest for explanation inevitably and rightly seeks for the ultimate explanation of everything observable.”

Cosmological Arguments can be found in the work of Plato (Laws Book X) and Aristotle (Physics Book II, Metaphysics Book IV) and make up the first four of Aquinas’ five ways to God in the Summa Theologica (1.2.3).   While Aquinas’ Cosmological Arguments are all framed as posteriori arguments – and so could never provide proof – they do provide strong support for the existence of a Prime Mover.  Nevertheless, Aquinas goes too far in his claim that this is what everybody calls God.

Aquinas’ first way draws on the Aristotelian concept of movement. In the Physics, Book V, Aristotle wrote, “all things that are in motion must be moved by something.”  Motion does not necessarily mean movement in the sense that things are is moving through space from location A to location B, but rather that they are moving from a state of potentiality to a state of actuality in multiple different respects. As Aristotle wrote, movement involves the… “actualizing of some potency. It is because things have real potencies that they are able to change.”  Aquinas later wrote, “for motion is nothing else than the reduction of something from potentiality to actuality.”  Whereas Plato’s argument, later refined by Muslim scholars of the 8th Century Kalam School and more recently by William Lane Craig, focuses on a temporal series of causes much like a domino-rally, pointing towards a beginning in time, an uncaused cause, which is what everybody calls God, for Aristotle and for Aquinas, even if the universe is as infinite as it appeared to be, there is still the need for a Prime Mover because everything depends on other things.  As Parmenides, Heraclitus and separately the Buddha observed, everything changes or moves and nothing stays the same, but nothing changes or moves without being moved by something else, even if that is just time itself.  As Aristotle wrote, “potential, precisely because it is potential, cannot make itself actual”.  Aristotle concluded that there must be a Prime Mover outside time and space, but stops short of claiming that this is God.  Aquinas went further, claiming that this Prime Mover is “what everybody calls God”, but in doing this he weakened the argument. It is true that the Prime Mover must be outside time and space and thus wholly simple and unchanging, pure actuality and with zero potentiality.  As Aquinas wrote,

“nothing can be reduced from potentiality to actuality, except by something in a state of actuality… it is impossible that in the same respect and in the same way a thing should be both mover and moved…” 

It is also true that everything ultimately depends on the Prime Mover for its existence.  As Aquinas wrote, “therefore it is necessary to arrive at a first mover, put in motion by no other”  However, it is a step too far to claim that the Prime Mover could be seen as the “creator”, let alone act in the world, speak to human beings or be crucified and rise again. When Aquinas writes “and this everyone understands to be God”  He goes beyond the evidence and possible knowledge.  In his book “The Nature of God” Gerard J. Hughes describes the Prime Mover changing potentiality to actuality in terms of a bowl of milk causing a cat to cross a room.  The bowl of milk does nothing, in the way that the Prime Mover – being timelessly unchanging and impassive – does nothing, because it has no potential and is pure act. It follows that Aquinas’ first framing of the Cosmological Argument provides strong support for the existence of a Prime Mover, but not for the existence of the God that Christians worship, because the Prime Mover would be unable to say “let there be light”, work miracles or judge individuals on the final day… all of these require in God potential and the ability to act in time, which the Prime Mover cannot have.

Aquinas’ second way draws on the Aristotelian concept of efficient causation.  For Aristotle, all things have four causes – material, formal, efficient and final.  Efficient causes are agents which bring things into being, in the way that parents bring their children into being or the earth, sun and rain bring the oak tree out of the acorn.  If everything depends on efficient causes to bring them into being, again there is a chain of causation which requires explanation.  The chain cannot be infinite, because if there was no first efficient cause there would be no subsequent causes and the universe would not exist.  Something cannot come out of or be caused by nothing. Similarly, there cannot be a first efficient cause like other things in the universe, as if there were it would need efficient causes of its own and could not, therefore, be the first.  Aquinas concludes, “it is necessary to admit a first efficient cause, to which everyone gives the name of God.” Again, Aquinas’ second Cosmological Argument provides strong support for the existence of an uncaused efficient cause and again, it is a step too far to claim that this must be the God of Christian worship.  This not least because efficient causes do not need to be sustaining causes in esse (as Frederick Copleston later called them) but could be a cause in fieri (again, to use Copleston’s terminology).   An uncaused cause which began the universe but has no further role in it is not the God of Christian theism; at most it supports deism.  Further, Aquinas admits that

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.”

 This implies that as efficient cause the uncaused cause must be PRIOR to the universe, something which would be difficult to reconcile with Big Bang Theory as this suggests that as time itself was created at the Big Bang, it makes no sense to speak of anything being PRIOR to it or indeed, as Stephen Hawking observed, causing it.  In these ways, Aquinas goes beyond the evidence in claiming that the uncaused cause is that to “everyone gives the name of God”.

Aquinas’ third way develops the idea of the contingency of things in the universe, pointing out that everthing has the potential to be or not to be; “We find in nature things that are possible to be and not to be”.  In an infinite universe, all potentials not to be might be expected to have been realized; as something can’t come out of nothing, nothing would then exist and I could not be writing this essay.  It follows, therefore, that EITHER the universe cannot be infinite – in which case there would have to be a first cause in time which would be what everybody calls God – or the universe is infinite and there exists a “necessary being”, a fully actual “neither something nor nothing” which contains its own explanation and has no potential not to exist.  This, Aquinas claims, is what “all men speak of as God.”  Aquinas’ third Cosmological Argument is just as problematic as an argument for the existence of the God of Christian worship as the first and second.  Not only as an a posteriori argument does it stop short of providing proof, it also goes well beyond the observable evidence in concluding that the necessary being is God as Christians would define Him.  Leibniz later recast the third Cosmological Argument as an a priori argument, writing:

“Why is there something rather than nothing? The sufficient reason […] is found in a substance which […] is a necessary being bearing the reason for its existence within itself.” 

For Leibniz, anything that exists has a cause for its existence, either in the necessity of its own nature or in an external cause.  If the universe has an external cause for its existence, this cause must be God.  As the universe exists, it must have a cause for its existence, either in the necessity of its own nature or in an external cause.  Because the universe exists contingently, not necessarily, the universe must have an external cause and this must be God.  Nevertheless, like Aquinas, Leibniz argument fails to prove the existence of the God of Christian worship.  Causing the universe is not enough to be called God; the Christian God does rather more than an abstract singularity or the Higgs Boson does.  Further, it doesn’t make sense to predicate much of what the Christian God does to the necessary cause of the universe supported by Aquinas and by Leibniz in his supposed improvement of Aquinas’ third cosmological argument.  Both the God of Aquinas and the God of Leibniz are timelessly impassive and it is inconceivable how such a being could act even once to create the world, given that this would involve a change in its being incompatible with being timeless and fully actual with no potential.  Further, as Immanuel Kant observed, we have no experience of necessary beings so it makes little sense to speculate about their possible existence.  Also, it is inconsistent to start an argument by claiming that all things are contingent and conclude by hypothesizing something that is not contingent.  Again, while Aquinas’ third Cosmological Argument strongly points towards the existence of a necessary being or beings in the universe, it is far from being conclusive proof of such, even when recast as an a priori argument and cannot in any case justifiably claim that the necessary being is what Christians worship as God.

While it is true that most of the classical criticisms of Aquinas presented by David Hume and Bertrand Russell fail to undermine his Cosmological Arguments, the point (which they all make) about the Cosmological Argument failing to support the God of Christian worship stands.

Firstly, Hume criticized a version of the Cosmological Argument presented by his character Demea in Dialogues Concerning Natural Religion (1779) Book IX.  Cleanthes points out that there is no support for the claim that everything in the universe is moves and is moved, caused and is caused or is contingent.  We have a limited view of the universe and no sensible reason to believe that the universe is homogenous or that we see things the way they actually are, a principle known in science as isotropy.  For all we know, argues Cleanthes, there could be unmoved movers, uncaused causes or necessary beings within the universe which could explain its continued existence.  Nevertheless, accepting these criticisms of the Cosmological Argument entails abandoning Natural Science altogether.  Leibniz coined the term “Cosmological Principle” to refer to the principles of homogeneity and isotropy which all scientists must assume in order to reason inductively towards natural laws.  Without the Cosmological Principle, we could not make many scientific knowledge claims; Cosmology and Quantum Science, Medicine and Biochemistry would all be a waste of time.  In practice, laws of nature supported by inductive reasoning enable mobile phones and space shuttles to work, so it doesn’t make sense to doubt the authority of our observations as Hume, through Cleanthes, does.  These criticisms of Hume’s fail to undermine Aquinas’ Cosmological Arguments insofar as they point to a necessary cause for the universe.

Secondly, Cleanthes continues by criticizing the claim that just because the parts of the universe have causes, so must the universe as a whole.  Demea (and Aquinas) rely on the so-called fallacy of composition.   Further, Cleanthes asks why the universe cannot be the explanation of itself, why there must be an external cause for the universe.  Later, Bertrand Russell asked why the universe cannot be a “brute fact”.  Yet neither of these criticisms is conclusive.  As Leibniz points out, it is difficult to see how a universe of contingencies can itself exist necessarily.  Contingencies involve potential which cannot, by definition, exist within a necessary being.  Further, while characteristics of the parts do not necessarily have to be characteristics of the whole and while (as Russell argued) just because all men have mothers it doesn’t mean that the human race has a mother, it can sometimes follow.  Each strand of spaghetti has two ends, something which also applies to the whole packet of spaghetti.  In a sense and because it is made up of material in the way that things in the universe are, the universe is a thing.  Things exist contingently and need to be moved and caused by things other than themselves.  These criticisms of Hume’s fail to undermine Aquinas’ Cosmological Arguments insofar as they point to a necessary cause for the universe as well.

Nevertheless, Cleanthes’ criticism that the cause of the universe could not be said to have the attributes of the Christian God is, for reasons previously explored, is persuasive.  Again it is clear that while Aquinas’ Cosmological Arguments do offer support to the hypothesis that there is an uncaused, necessary cause for the universe, they are far from proving that the God of Christian worship exists. 

In conclusion, Aquinas’ Cosmological fail to prove that God exists, both because as a posteriori arguments they stop short of proving their conclusions and because even if they are reframed into a format which could provide proof, as Leibniz attempted, they demonstrate only the existence of an abstract necessary being far short of having the attributes of the God Christians worship. 



  • Class notes on the Cosmological Argument
  • Aristotle, Physics Book V
  • Aquinas, Summa Theologica 1, 2, 3
  • Hume’s Dialogues Concerning Natural Religion Book IX
  • Vardy & Vardy “God Matters” Chapters 4 & 5

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