Academic Philosophy

The Five Proofs of God of Thomas Aquinas

  • For the bachelor course 'history of medieval philosophy' of the Radboud University Nijmegen


When we subject the term 'proof of God' to an analysis we see that it is composed of the two words 'God' and 'proof'. Semantically seen is it remarkable that these two words have come to a combination. For 'God' makes us think spontaneously to faith and religion where 'proof' makes us think more to logic and empirical science, and these are two very different ways of approach to truth and reality. That the philosophical proofs of God have flowed mainly from the pens of thinkers of the middle ages gives a clear reference to that with which the thinkers of the middle ages mainly occupied themselves; the uniting of the antique philosophy, which was grounded in logic and empiricism, with the Catholic faith.1 And this they did, dependent on time and person, with on the one side the Platonic emphasis on the ratio and on the other side with the Aristotelian emphasis on empiricism.2 Thus have also the middle ages brought forth two basic types of proofs of God whereby as base either the ratio or empiricism was taken. The best known proof of God with as point of departure the ratio is probably that of Anselm "We believe that Thy art something above which nothing bigger can be thought" of Canterbury (1033-1109).3 This inventive proof of God from his 'Proslogion' we are not going to discuss here. The proofs of God that here shall be discussed are the five proofs of God of Thomas Aquinas (1225-1274). And we shall see that Aquinas takes as point of departure (Aristotelean) empiricism. With this are the proofs of God of Aquinas, which are contained in the Summa Theologiae (Book I, Quaestio 2), to be called exemplary for the whole of his doctrine because in that doctrine he made aspects of the Aristotelian thought merge in that of the Catholic church, or at least tried to do this.4 This by the way to the pleasure of the church, because although he was in his time except respected still also controversial would that controversy in later times clear space and even give way for an outspoken celebration, concretised in the praising words of Pope Leo XIII.5 We in contrary shall suspend our praising explications for the moment and move over to a more objective consideration of Aquinas's five proofs of God.

The Articles of the Five Proofs of God

Article II: Whether the Existence of God is Demonstrable

As introduction to the proofs of God is this a worth mentioning article because it shows explicitly that Aquinas as point of departure does not handle the ratio but empiricism. Unknown causes, according to Aquinas, can become demonstrable from their known effects. Thus must God as unknown cause be provable from the to us known effects.6

Article III: Whether God Exists

This article, under which also the five proofs of God are worked out, has a remarkable introduction. Against the supposition that God does not need to exist necessarily does Aquinas conclude on base of the Biblical expression about the person of God; "I am that I am" that in nature of things a first unmovable being must be found, a first cause, which is God.7 A non-Christian philosopher would probably accept neither such an assumption nor such a conclusion on base of that assumption as valid. What this introduction shows us is that Aquinas, despite the handling of an Aristotelean thought (which in the form of 'God as unmovable being and first cause' is also contained in this conclusion), still handles the Holy Scripture as first authority and as trustable supplier of assumptions.

The Five Proofs of God

Proof of God 1

The first proof of God departs from movement and change. All things on Earth undergo change. Change does Aquinas understand Aristotelean as the bringing forth from potency to act. Now according to Aquinas can nothing be brought from potency to act than through something actually existing. And because the same cannot be at the same time potency and act is all movement brought forth by another actual existence. But also this actual existence is being brought into movement by something else, en so on and on. However this cannot continue endlessly. For then there would be no first mover and because of that no movement at all. Because the successive things would not move but through what is moved by the first mover. Therefor it is necessary to return to a first mover that is moved by nothing else, and this one every human knows as 'God'.8


Aquinas uses in this proof of God his conclusion (that there must be a first unmoved mover) as argument in his argumentation for proof. This he does when he states that without first mover there would be no movement at all and that the successive things would not move except through what is moved by the first mover. The assumption that a first mover is necessary for movement is a hidden assumption in his reasoning which is also his conclusion. He is situated thus in a circle reasoning.

Proof of God 2

This second proof of God looks like the first but instead of to depart from the goal cause does Aquinas here depart from the work cause. Nothing can be work cause of itself, because then it had to exist before itself, which is impossible. Now also here can the shackles of the work causes not endlessly go on. Without first work cause could there be neither a first effect nor other work causes. Thus we must assume a first work cause, which all call 'God'.


Here we find a same type of circle reasoning as with the previous proof of God. The importance of the assumption of a first effect is only valid with the assumption of a first mover. Aquinas accepts that importance silently, and thus does his argumentation contain a hidden assumption which is the same as his conclusion.

Proof of God 3

The third proof of God is built on the notions of possibility and necessity. We experience that there are things that both can exist and not, that thus come to being and perish and that thus exist or not exist. Now it is impossible for such things to exist eternally, for what has the possibility to not exist shall once also be non-existent. And when everything has the possibility to be not, then among things there once was nothing. And if this is true then even now there would be nothing, for what not exists does not come to being except through something that already exists. Now if everything once was nothing then it was impossible for something to come to being, and that would mean that also now there would be nothing, which clearly is not correct. Thus there must among the things, besides contingent things, exist also a necessary thing. Now necessary things can have either no cause or a cause of somewhere else. For that necessary things that have a cause of somewhere else the chain of causes cannot go on endlessly. Thus there must be a necessary existence which itself is causeless and itself is the cause of all necessary existing things, which all call 'God'.9


In this proof of God does Aquinas apply his in the meantime well known circle reasoning twice. The first time he does this when he states that when everything has the possibility to be not therefrom it follows that once there was nothing. This is only a valid conclusion when the assumption is accepted that the chain of causality of existence cannot be traced back endlessly. So this assumption is hidden here in Aquinas's argumentation. The second application of his circle reasoning is more explicit. He clearly states that with necessary things that have a cause of somewhere else the chain of causes cannot go on endlessly. Again an assumption which he actually makes groundlessly, but which does makes possible his proof of God.

Proof of God 4

The fourth proof of God, the proof of God of the degrees, takes as point of departure that in things can be found more or less goodness, truth, nobleness and such. But 'more' or 'less' are terms that are applied to diverse things for as far as they near that which has those qualities in the highest degree. So there must exist something which is the truest, the best, the noblest and thus also the greatest being. Now what in a kind is considered as the highest is also the cause of everything which belongs to that kind. Thus there exists something which is the cause of the existence of all things and of the goodness and of all perfection, and this we call 'God'.10


Terms like 'more' or 'less' with individual things need not necessarily be in relation with a thing on which a term like 'the most' is applicable. They can just as well be considered as having reference to relations between the individual things themselves. Also it is absolutely not evident that the thing that has a certain quality in the highest degree must be cause of all other things with that quality. One can accept it as assumption, and this Aquinas does, but it is in this proof of God certainly no valid reasoning.

Proof of God 5

The fifth proof of God conclusively comes to being from the order of things, for we see that some things which lack reason are still working in relation to a goal. This becomes clear from the given that they are always or mostly working in a same way and do what is best. From this shows that they do not achieve their goal by accident but from a directedness. Now things that do not have intelligence do not tend to a result except when directed by a knowing and intelligent someone. Thus there is an intelligence which orders all natural things in line with a plan, and this we call 'God'.11


It seems evident that when reasonless things are usually working in a same way it does not necessarily have to show that they achieve a goal from a directedness. Aquinas hides here the assumption that the working of reasonless things have a goal whereby he on base of that can conclude that they achieve their goal through the direction of a knowing and intelligent someone.


Generally considering Aquinas's proofs of God we see that he every time comes to his conclusions through hiding assumptions in his argumentation. Every time is the conclusion then that there must be a necessary 'something'. But after that does Aquinas take another remarkable step. For that 'something' he equals every time to God, with which he naturally means the Catholic Biblical God. Here he does skip a step. For one could expect that he by Biblical texts would show why that necessary something of his conclusion can be equalled to the Biblical God. Did Aquinas consider the Biblical texts that could have made that bridging possible perhaps as too evident to mention, or do we have to do here with an unbridgeable gap? Now it is here of course not the place to let a dialectic battle flare up again or to take a clear position therein. The gap between Aquinas's God and Aquinas's proof does bring us back to the introduction where the term 'proof of God' was characterised as a remarkable combination. Aquinas has not been able to replace that remarkability with a plausibility. His argumentation has too many hiatuses for that, under which the mentioned gap. We do however stay happily open to unbiasedly consider new attempts of the proving of Gods existence.

  • Arthur Hyman & James J. Walsh (editors), Philosophy in the Middle Ages, The Christian, Islamic, and Jewish Traditions, Hackett Publishing Company, Indianapolis / Cambridge, 1973.
  • David Luscombe, Medieval Thought, Oxford University Press, New York, 1997.
  • Oliver J. Thatcher (editor), The Library of Original Sources, Volume V, The Early Medieval World, University Research Extension, Milwaukee, 1907.