Saturday, 17 October 2009

Q2: The Existence of God

Why this Question Matters.

Having established the nature and scope of sacred teaching in the first question, Aquinas turns to the question of the existence of God. It is important to note that here and in the next few questions, he is concerned with the existence and nature of God as known to natural reason (following St Paul in Romans 1:19-20). How God is known from revelation and the relationship between natural and revealed knowledge of God will follow later. The third article of this question lays out the famous quinque viae (five ways); these are five arguments from natural reason to prove the existence of God.

The Thread of the Argument

A1: Aquinas asks first whether the existence of God is self-evident. Three objections in turn suggest the following: that knowledge of God is implanted in us by God and is therefore self evident; that God being “that than which nothing greater can be conceived”, He must exist, as to exist is greater than not to exist. (This is the so-called ontological argument of St. Anselm.); and that God being truth itself (John 14.6), His self-evident existence follows from the self-evident existence of truth.

In answer, Aquinas makes a distinction between propositions that are self-evident in themselves but which are not known to us because we do not know what the terms of the proposition really mean, and propositions which are self-evident in themselves and known to us because we do. As we do not really know what God is, the proposition that God exists is placed in the former category. In Ia.q3.a4, Aquinas will demonstrate that God is His own existence, therefore the subject and predicate in the proposition “God exists” are identical; therefore the proposition is self-evident in itself. However, the proposition is not self-evident to us and has to be demonstrated by sophisticated arguments based on those of God’s effects that are known to us. This latter Aquinas will outline in the third article.

The failure of the first and third objections is now clear by means of the distinction. The ontological argument is rejected on the two grounds that it is unclear that the characterisation of God is correct and that also that the reasoning is faulty. Aquinas demands that for the argument to work we must in fact know that “that than which greater can be thought” must exist; which rather ruins the argument.

A2: Before actually presenting arguments for God’s existence, Aquinas asks the more fundamental question of whether it makes any sense to try to demonstrate God’s existence. The objections suggest that the existence of God is an article of faith and that it makes no sense to demonstrate matters of faith; that we don’t know what God is and therefore we can’t demonstrate God’s existence; and that as all we could naturally know of God arises from his finite effects (i.e. those known to us) we could not know about the infinite God.

Aquinas replies that there are two types of demonstration: from cause to effect and from effect to cause. The first type of demonstration answers questions of the form “why is something the case”; demonstrations of the second sort answer questions of the form “that something is the case”. Demonstrations of God’s existence are of the second type (Romans 1:20): they can tell us that there is some cause causing an effect even if we can’t infer much about the cause itself. The second and third objections fall immediately. To the first objection Aquinas points out that faith presupposes natural knowledge, so a natural demonstration of God’s existence (which faith reiterates, or upon which faith may build) is reasonable.

A3: After having argued that, in principle, one can in fact construct demonstrations for the existence of God, Aquinas proceeds to do so. He presents the famous quinque viae (five ways).

Before he gets under way two objections are presented which are objections to the very existence of God. The first is a version of the argument from the existence of evil; God must surely be infinite Good and if He exists then evil could not. The second uses what later became known as Ockham’s Razor; we can explain the world in terms of natural causes therefore there is no reason to posit the existence God. The first objection is countered with the observation that God can permit an evil so as a good may come from it; the second is rejected because Aquinas denies that we do get a full explanation of creation from within itself.

The first of the five ways argues that anything changing is being changed: its potentiality is being actualized by something external to itself. But that something is itself being moved from potentiality to actuality by something external; and so on. However, the chain of agents cannot be infinite, therefore there is a first agent that cannot itself be being moved from potentiality to actuality, therefore it is pure actuality and we call it God.

The second of the five ways is similar in form to that of the first way but applies the reasoning to “per se” series of efficient causes to infer that there must be a first efficient cause.

The third way observes that if there was ever a point at which absolutely nothing existed, then nothing could ever exist, because things depend on other things for their coming to be. Aquinas goes on to infer from this that there must be some thing, the existence of which is necessary (rather than contingent). Another infinite regress argument (this time on necessary things receiving their necessity from other necessary things) arrives at a first necessary thing which we call God.

The fourth way argues that the existence of different grades of perfection (with respect to various properties) in created things points to a most perfect thing with respect to such properties, which we call God.

Finally the fifth way observes that non-intellectual things act for an end, from which Aquinas infers that some intelligence, which we call God, must be supplying that end to them.

Summary and Handy Concepts

  • Something may be self-evident in itself, in the sense that the predicate is implied by the subject’s definition, but not known to us because we don’t have a true understanding of the terms that express that thing. The proposition “God exists” is a proposition of this sort.
  • Demonstrating why something is the case (from cause to effect) and demonstrating that something is the case (from effect to cause) provide two different approaches to the notion of demonstration. Demonstrations of the existence of God based on natural reason are demonstrations of the second type.
  • Aquinas believes that our only natural access to God is through our sensory experience.
  • The division of being into actuality and potentiality, with God the only being that is pure actuality, is absolutely fundamental in Aquinas’s thinking about God. He will use this idea throughout the summa and especially in the questions about God. In recognition of this, the very first of the twenty-four Thomistic theses approved by the Sacred Congregation for Studies in 1916 is dedicated to this division of being.
  • For more details of the metaphysical assumptions underlying the five ways, please refer to the section on Metaphysics.


  • Given the enormous attention paid to the ontological argument over the ages, Aquinas is really quite dismissive of it. This reaction may very well arise from Aquinas’s belief that we can only naturally approach God through created causes. This approach means that even framing the concept of “God” in the way that St Anselm frames it needs some preparatory work; how do we know that St Anselm’s definition is coherent, for example?
  • The objection to the existence of God based on the existence of evil is also despatched very quickly. For those wanting in depth treatments of the so-called “problem of evil”, two books by Brian Davies are highly to be recommended: The Reality of God and the Problem of Evil and Thomas Aquinas on God and Evil.
  • It’s clear that the five ways were well known to the audience of the summa, as the treatment is very brief. The brevity of our summary above reflects the brevity of the arguments as presented by Aquinas! But Aquinas takes a lot of metaphysical assumptions for granted that we have to recover from other sources and an adequate treatment of any of the five ways takes many pages of careful argument. One of the best and most accessible modern accounts of the five ways is given in Edward Feser’s book Aquinas. Aquinas himself goes into much more detail about natural theology in the summa contra gentiles.
  • The reader must be warned that there are many terrible accounts of the five ways out there. The common feature of many of these is to attempt to interpret the terms used in the arguments as they tend to be used in modern philosophy (or indeed, without any recourse to philosophy at all). For example, the modern notion of a cause is radically different from that assumed by Aquinas.
  • In the third way, Aquinas infers the existence of (at least one) necessary being by an argument that, on a first reading, appears to commit what is called a quantifier shift fallacy. He appears to argue that because for each thing there is a time when it must not be, therefore there is a time when all things must not be, which is clearly fallacious. Here, perhaps, is one of those rare occasions where an argument ad hominem is justified: Aquinas simply would not have made such an elementary slip, so another explanation must be found. Perhaps, as Freddoso argues, Aquinas might be read as hypothesising that if everything is subject to generation and corruption then there must have been a time when there was nothing, as everything includes the prime matter out of which material things are made. For another argument, refer to Feser op. cit.
  • The fourth way, from the grades of perfection in creation, is generally held to be the weirdest for modern people to get their heads around. Indeed, it is possible to look at what Aquinas has written and to ask where the argument is. Again, the account that Aquinas gives does not really do justice to the subtlety of the argument.
  • Similarly, Aquinas’s account of the fifth way is disappointingly short especially as this may very well be the most profound of the five ways.

Revised March 2012


  1. If I understand correctly, Freddoso is saying that if everything is subject to generation, something which is not everything had to generate the first thing that is included in everything and from everything (else) traces its lineage. Given the premise that "everything is subject to generation," which is a fair statement about the world based on our experience of it, then it does follow that you need a point at which a non-generated generator (forgive me) must do the first generating and that generator is not "everything" or a perhaps is not classed among "everything." Fair enough; is God part of everything? If so, the argument falls, it seems to me. This must be more than a matter of usage because we can use the term "everything" to include or exclude God depending on our point. There has to be more than a semantic definition of "everything" at play here. Can we define God as that which is more than and outside of "everything?" I think that pretty much would fit with the traditional Western approach to what the word God means. (It would not sail in India though.)

    1. God isn't a part or a thing, therefore He isn't a "part of everything". He isn't a part, nor is He the sum of the parts, but He is the Absolute from which they all come and from which they all retain their being.

    2. But God exists in everything as cause of existance. Q8 is dealing with it.

    3. But God exists in everything as cause of existance. Q8 is dealing with it.