Saturday, 17 October 2009

Q3: God's Simplicity


The second question has just shown us that we can demonstrate “that God is”. However full answers to the questions implied by “what God is” remain hidden to us; comprehension of the divine is simply beyond our intellects. This being so, it is still possible to say things about God that are true statements; especially we can say things about what God is not. Aquinas will be taking this negative (or apophatic) approach to questions about God from here until question thirteen using what is sometimes called the via remotionis (the way of remotion) or the via negativa. Questions twelve and thirteen discuss how we can know about God and what we can say about Him. They act as a transition into the next section of the summa that deals with what we can positively affirm about God, the so-called via affirmationis (the way of affirmation).

Why this Question Matters.

The Doctrine of Divine Simplicity (DDS) expresses simultaneously one of the strangest, hardest and most important facts about God; that in Him there is no composition. Its importance explains why Aquinas considers it so early on in the summa. As an analogy, in mathematics it is not uncommon to prove a highly general, abstract, theorem about which it is very hard to build an intuition; this general theorem is them applied to a number of special cases, reeling off consequences with relatively little additional effort. The DDS is rather like this; having put in the effort to demonstrate it, many consequences about God follow relatively easily. For example, later on Aquinas will use the DDS to show that since scripture tells us about relations in the Godhead (for example, Fatherhood), they must be substantial subsisting relations corresponding to the persons of the Divine Trinity.

Aquinas’s strategy in this question is to consider in turn the different ways in which there might be composition in God:

  • Is He made up of material parts?
  • Is He a composition of form and matter?
  • Is He a subject bearing a nature or essence?
  • Is He a composition of essence and existence?
  • Is He in a genus, differentiated by species?
  • Is he a composite of substance and accidents?

In each case, Aquinas shows that such a composition is impossible in God.

The Thread of the Argument

A1: Aquinas starts with the simple question of whether God is a body (in the sense of being an extended three-dimensional thing in the world). The objections, suggesting that God can be considered a body, are all based on scriptural descriptions that use bodily terminology (and are contrasted with the sed contra from John 4:24 that “God is spirit”). Aquinas offers three proofs that God cannot be a body: an unchanging first cause of change cannot be a body; God, as pure actuality has no potentiality, whereas all bodies do; and the “excellence” of a body is derived from an external principle (e.g. the soul in the case of our bodies) whereas God depends upon no external principles. Most of the objections are answered by observing that scripture sometimes uses metaphorical and figurative language when talking about God.

The second objection argues that as man is made in the image and likeness of God (Genesis 1:26) then God must be corporeal as we, His image, are. In answer to this particular biblical image, Aquinas makes a point that he will frequently repeat. It is because we are rational intellectual creatures that we are in His image, not because we are bodily creatures.

A2: Even if God is not a body, might He still be composed of form and matter? No, because in-formed matter still has potentiality; God does not. Likewise, composites of form and matter only participate in goodness (i.e. they bear the form of goodness) rather than being goodness itself like God. Thirdly, all agents (i.e. things that perform actions on other things) act as agents depending on their type of form; the first agent (i.e. God) is essentially an agent therefore must be essentially form and therefore not a form-matter composite.

The third objection suggests that God must have matter, and must therefore be composed of matter and form, because matter is what individuates things. (This dog is not that dog because it is made of a different lump of matter even though they both participate in the same form). Aquinas denies that all forms have to be received in matter. In doing so, he points out that such self-subsisting forms (like God and the angels) are individuated simply because they are not received into a subject.

A3: Aquinas next asks whether God can be considered a subject bearing an essence. (The essence of something is what makes the something what it is, and a subject is something that can receive an essence.)

The objections appeal to what seems to be common sense. For example, things in the world are not the same as their natures (a particular human is not humanity, for example). Similarly, God is the cause of creatures and causes resemble their effects, therefore God must be a subject bearing an essence.

Aquinas’s reply gives us the first major indication that he considers God as radically different from His creation: no, He is not a subject bearing an essence. Aquinas notes that for things composed of form and matter, what makes them what they are as things is not associated with their individuality but is associated with their species. Roughly speaking, form is associated with species and matter with individuation. But for things like God that are not matter-form composites, individuality cannot be derived from matter, so their forms are intrinsically individual. (Recall that we saw this argument in the reply to the third objection in the second article.) Therefore such things coincide with their natures. Aquinas immediately draws the radical corollary that God coincides with anything truly predicated of Him: He is His divinity; He is His own life and so on.

In replying to the first objection, Aquinas makes the important point that our language is adapted to talking about composite things. If we want to talk about something that is not a composite, we do so by analogy using concrete and abstract nouns to talk about different aspects of such things. This should not fool us into thinking that such linguistic differentiation implies real differentiation in the simple things themselves.

A4: Having shown that God is His own essence, Aquinas now shows that He is His own existence, thus justifying the description ipsum esse subsistens (self-subsistent existence) of God. To support his claim, Aquinas gives three arguments.

The first argument claims that if the existence of a particular thing is distinct from its essence then its existence must have a cause external to its essence. But this is a clear contradiction in the case of God as He is the first efficient cause. The second argument is similar, based on the potentiality that must be in something where essence and existence are distinct. The third argument takes a different, somewhat neo-Platonic, course. If in God, essence and existence are distinct then God can only be participating in existence. This contradicts God’s being the primary existent.

A5: Aquinas now asks whether it makes any sense to apply the notions of genus, difference and species to God. The sed contra expresses what one might anticipate given Aquinas’s treatment of God so far: the notion of genus is logically prior to the things that exemplify it, but nothing is prior to God (in any sense) therefore God cannot be allocated a genus.

Given the apparent power of such an argument, the length of Aquinas’s reply might seem surprising. But Aquinas is being careful to note that the concept of genus applies to a wider class of things than to just concrete objects in the world (for example, to mathematical objects or to privations) and he wants to make sure that all the bases are covered. He offers three arguments.

First, Aquinas claims that species arise within genii by means of the actualization of some potentiality that gave rise to the generic notion common to the genus. He gives the example of rationality arising out of the sensory, differentiating the human species from the non-rational in the genus animal. As the actualization of potentialities does not occur in God, this cannot be applied to Him.

Second, as the genus of something expresses its essence, and God’s essence is His existence, the only possible genus for God would be that of being; but Aristotle has already show that such a genus cannot exist.

Third, a genus expresses the essence of its members, but the individual members of the genus within species have their own distinct existences distinct from the genus. Therefore this cannot apply to God.

A6: As accidents realize some potentiality of their subject and as God is pure actuality, it is no surprise that God cannot be a composition of substance and accidents. Aquinas gives two other arguments about this: firstly, one cannot add anything, such as an accident, to something that is its own act of existence; secondly, as all derivation starts from God, there can be nothing derived in Him.

A7: As a sort of summing up, Aquinas states the question to which all the preceding articles have been leading: having considered all the different ways in which things can be composites, is God Himself a composite of any type or is He entirely simple?

Aquinas puts forward two objections. On the one hand, a cause will resemble its effects and all created things, caused by God, are composites of some sort; therefore God must be a composite of some sort. (This generalizes the argument of the second objection to the third article above.) On the other hand, in the world of which we are aware composite things are better than simple things, therefore we should not attribute simplicity to the best of all possible things. For the moment however, Aquinas leaves the answers to these objections until later in the summa (Ia.q50.a2 ad 3 and Ia.q4.a2 ad 1 respectively).

In his response to the question, Aquinas summarizes the articles that have gone before showing that God is not a composite in the ways considered. He also adds some additional observations: a composite is subsequent to its components, but God is not subsequent to anything; the composition of any composite is caused, but God is the uncaused first cause; in any composite there must be a mixture of potentiality and actuality, but God is pure actuality; and finally, within a composite there are things that are not the composite itself, but God is pure form in which this cannot be the case.

A8: Finally, having disposed of the notion that God is a composite considered in Himself, Aquinas considers the possibility that He might enter into composition with other things. Aquinas’s major concern in this question seems to be to refute three major errors: that God is the soul of the world; that God is the form of all things; or that God is prime matter. Each of these involves God being a component of some sort of composite.

Aquinas’s response (although it could just reiterate what he has said already) appears to be tailored to the motivations behind the erroneous propositions. So he argues that a first efficient cause may resemble its effects but need not be numerically equal to them; components cannot be primarily and essentially efficient causes in the way that God is; and a part of a composite cannot be the first being, as form, when part of a composite, participates in that something rather than being that something essentially.

Handy Concepts

  • Everything is either pure actuality (i.e. God) or a mixture of actuality and potentiality.
  • God is not a body made up of parts because such a composite would retain un-actualized potentiality.
  • God is not a composite of form and matter.
  • Something participates in a form when if it bears the form but is not identical with it.
  • The second argument in the second article is going to have profound implications when generalized later on. Aquinas differentiates between being a such-and-such by participation (I am a human being because I bear the form of a human being) and being a such-and-such by essence (God is good by His very nature). When this distinction is applied to the notion of being itself, we arrive at the idea of God as ipsum esse subsistens (self-subsisting being) as in the fourth article.
  • The essence or nature of a thing is what makes that thing the sort of thing that it is. A subject is what receives or bears an essence. God is not a subject bearing an essence.
  • God coincides with anything truly predicated of Him.
  • God is self-subsisting existence (ipsum esse subsistens).
  • God is prior to any notion of genus and therefore is not in any genus, let alone differentiated as to species.
  • A substance is a concrete particular thing that bears attributes. Accidents are attributes of things that are not essential to their identity. God is not a composition of substance and accidents.
  • God is not a composite in any sense, nor does He enter into composition with other things.
  • Davies, in Thomas Aquinas on Good and Evil, helpfully sums up the notion of the simplicity of God as affirming that God is (i) not changeable, (ii) not an individual belonging to a natural kind, and (iii) not created.
  • The doctrine of God’s divine simplicity is not original to Aquinas; indeed it was taught as a doctrine of the faith by the fourth Lateran Council before Aquinas was born.


  • Why is it that God’s simplicity is the hardest thing to write about?
  • In the third article, Aquinas makes the claim that effects resemble their causes. This is a claim that will be repeated many times throughout the summa; especially in the form omne agens agat sibi simile (every agent makes its like). This may seem rather odd, but what Aquinas means by this is that an agent cause expresses something of its own nature in the effect that it has on a patient as it is actually having the effect. So a vat of acid doesn’t look like a dissolving piece of metal but as the acid is at work dissolving the metal it expresses something about itself in the very act of dissolving. This should remind us that Aquinas thinks of causes in very immediate terms: he’s not just interested in the fact that a virus causes a cold; he’s interested in the way in which the virus acts in causing the cold.
  • The idea that created things are composites of essence and existence is a controversial issue at the foundations of metaphysics. Aquinas nailed his colours to the mast very early in his career with the work de esse et essentia.

Revised March 2012

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

Introductory Books

"Of making many books there is no end, and much study is a weariness of the flesh" (Eccl 12:12)

There are many books about Aquinas, about his thought and about his intellectual and spiritual impact throughout history. Here are some suggestions for books that I have found very helpful for covering the basics of Aquinas's thought in clear language.

  • Edward Feser, "Aquinas: A Beginner's Guide", OneWorld, ISBN 1-8516-8690-8

This has now become my first recommendation for a book introducing Aquinas’s thought. Although this is entitled a "beginner's guide", it does assume slightly more background in the history of philosophy than the two books by Brian Davies that we mention below. Making this assumption allows Feser to connect Aquinas's thought with other philosophers (especially of the early modern and contemporary periods). From this book, you'll get a good idea of how the great tradition of metaphysics was rejected (without all that much in the way of rigorous refutation) in the early modern period and how it is making something of a comeback today.

  • Brian Davies, "Aquinas", Continuum, ISBN 0-8264-7095-5
  • Brian Davies, "The Thought of Thomas Aquinas", Clarendon Press, ISBN 0-1982-6753-3

Brian Davies is an English Dominican who is currently professor of philosophy at Fordham University in the USA. There's a lot of overlap between these books, but both of them are excellent and if you like one, it's worth getting the other! Davies will give you more background than Feser, but he also approaches Aquinas as a discussion partner from the point of view of modern analytic philosophy in the Anglo-American tradition. You may find this a strength or a weakness!

  • Fergus Kerr, "Thomas Aquinas: A Very Short Introduction", Oxford University Press.

I have to admit that I haven’t read this book, but anything written by Fergus Kerr is worth reading! In my experience, the OUP "Short Introductions" tend to be pretty useful. And it's cheap!

  • GK Chesterton, "St. Thomas Aquinas" (many editions).

A classic, with the style that only GK could muster! Still worth reading today.

We've given links to a couple of online translations of the summa. In the sessions we're generally using the Freddoso translation backed up with the English Dominican translations. It's also worth mentioning:

  • Brian Davies and Brian Leftow (eds.), "Summa Theologiae, Questions on God", Cambridge University Press.

This gives a good modern translation of Questions 1-26 (omitting questions 16, 17, 23 and 24), improving (in our opinion) on the corresponding volumes of the later English Dominican translation.

  • Timothy McDermott (ed.), "Summa Theologiae, A Concise Translation", Christian Classics.
This compresses the most important bits of the summa into a single volume. The ambition of this book is extraordinary and I think it’s a tribute to the author to say that he often succeeds in distilling the essence of the summa. However, I’ve found some of the choices for inclusion and omission to be slightly frustrating and the translation a little bit loose in places. Definitely worth having to hand, but the reader should be prepared to dive into the full text in places.

Another translation worth mentioning is that published by the Aquinas Institute ( This provides an update to the earlier Dominican Translation, and is published as a parallel Latin/English text.

NB: Edward Feser has also published a robust philosophical refutation of the ideas of the "new atheists" in his volume "The Last Superstition" (ISBN 1-5873-1451-7). One might note that the tone of this book is quite "bare knuckle" - not attempting to spare the feelings of his opponents. It is, however, the most accurate and accessible refutation of those opponents that I know of and a fine introduction to the metaphysical and theological thought of Aquinas.