Aquinas’s Argument from Contingency for the Existence of God: Introduction

This is the first post in my third series looking at arguments for the existence of God. I finished the last one a few months ago, which focused on Aquinas’s Second Way, or the First Cause argument (to read those, click these links: Part 1, Part 2, Part 3, Part 4, Outlined Version). Before that I looked at Aquinas’s First Way, or the Prime Mover argument (Part 1, Part 2, Part 3, Outlined Version). This new series will be examining Aquinas’s Third Way, or the Argument from Contingency for the existence of God.

Before we begin, however, it is necessary to look at relevant contexts and concepts for understanding the argument. The Third Way, along with the previous two ways, are all cosmological arguments. A cosmological argument is not so named because it has anything to do with the modern scientific field of cosmology; rather it comes from the greek word kosmos which refers to the existence and order of the world/universe. Philosopher William Lane Craig offers this simple definition:

“the cosmological argument is an a posteriori argument for a cause or reason for the cosmos” [1].

Elsewhere he writes:

“The cosmological argument is a family of arguments that seek to demonstrate the existence of a Sufficient Reason or First Cause of the existence of the cosmos. The roll of the defenders of this argument reads like a Who’s Who of western philosophy: Plato, Aristotle, ibn Sina, al-Ghazali, Maimonides, Anselm, Aquinas, Scotus, Descartes, Spinoza, Leibniz and Locke, to name but some. The arguments can be grouped into three basic types: the kalam cosmological argument for a First Cause of the beginning of the universe, the Thomas cosmological argument for a sustaining Ground of Being of the world, and the Leibnizian cosmological argument for a Sufficient Reason why something exists rather than nothing” [2].

Of the three categories provided here, the one which is obviously most relevant to us is that of Thomistic cosmological arguments; the first three ways all fit within this category. A small number of people might also include the Fourth Way; but it is significantly different enough so that most Thomists would not consider it a cosmological argument.

Of all the cosmological arguments, and even of all the arguments for the existence of God in general, the kalam is definitely the most popular today, as I’ve mentioned in earlier posts. The kalam focuses on the physical beginning of the universe, and as such has made use of Big Bang cosmology. There are two main branches of defense for the kalam, and two main branches of objections (from what I have encountered so far). The first main branch of defense comes from evidence for a physical beginning of the universe from contemporary Big Bang cosmology. The second is a philosophical argument based on the impossibility of an actual infinite, and thus the impossibility of an actual infinite past, which suggests that the past must be finite. The first main branch of objections argues that Big Bang cosmology does not necessarily require an actual beginning to the universe, or at least that this has not been fully settled yet. The second argues that the kalam rests on a specific theory of time, which it then proposes is the incorrect view.

The second most popular cosmological argument is less commonly known overall than other theistic arguments besides the kalam (such as the fine tuning or moral arguments), but still enjoys relatively frequent attention, compared to Thomistic arguments which are almost completely ignored today: the Leibnizian cosmological argument. The Leibnizian cosmological argument is also known as the Leibnizian contingency argument, or just the argument from contingency. This is relevant to us because Aquinas’s Third Way also argues “from contingency,” but in a different sense, as we will come to see.

So, in the rest of this post we will 1) look at further differences amongst types of cosmological arguments, 2) examine in further detail the Leibnizian contingency argument, and 3) discuss how it relates to and differs from the Third Way.

Philosopher Alexander Pruss gives another definition of a cosmological argument:

“A cosmological argument takes some cosmic feature of the universe — such as the existence of contingent things or the fact of motion — that calls out for an explanation and argues that this feature is to be explained in terms of the activity of a First Cause, which First Cause is God” [3].

Pruss chooses to categorize types of cosmological argument along similar lines as Craig, although basing such categorization on the specific way the distinct types handle a set of key problems for cosmological arguments in general:

“A typical cosmological argument faces four different problems . . .” [4].

The four problems are first, the question of whether these features of the universe which might call for explanation are actually open to explanation at all; second, what Pruss calls the “Regress Problem,” or “the problem of how to deal with an infinite regress of causes or explanations” [5]; third, the question of how to explain that which is posited to explain the certain feature of the universe, the extremely simplistic and popular version of this problem being the “What caused God?” question; and fourth, the problem of how to get from the First Cause to God.

Pruss says:

“the best way to classify cosmological arguments is by how the address these problems. There are then three basic kinds of cosmological arguments: kalam, Thomistic, and Leibnizian. The kalam and Thomistic arguments posit an intuitively plausible Causal Principle (CP) that says that every item of some sort — for example, event, contingent being, instance of coming-into-existence, or movement — has a cause. The arguments then split depending on how they handle the Regress Problem” [6].

With this having been laid out, let’s take a look at some common formulations of the Leibnizian cosmological argument. Pruss’s basic, very general outline of it goes like this:

  1. Every contingent fact has an explanation
  2. There is a contingent fact that includes all other contingent facts [i.e., the universe]
  3. Therefore, there is an explanation of this fact
  4. This explanation must involve a necessary being
  5. This necessary being is God [7].

William Lane Craig’s version looks like this:

  1. Anything that exists has an explanation of its existence, either in the necessity of its own nature or in an external cause.
  2. If the universe has an explanation of its existence, that explanation is God.
  3. The universe exists
  4. Therefore, the universe has an explanation of its existence.
  5. Therefore, the explanation of the existence of the universe is God [8].

Both of these make either implicit or explicit use of the concepts of contingent and necessary beings, from whence the “contingency argument” derives its name. These are technical terms in contemporary philosophy:

“It is commonly accepted that there are two sorts of existent entities: those that exist but could have failed to exist, and those that could not have failed to exist. Entities of the first sort are contingent beings; entities of the second sort are necessary beings” [9].

Craig’s definition:

“there are two kinds of being: necessary beings, which exist of their own nature and so have no external cause of their existence, and contingent beings, whose existence is accounted for by causal factors outside themselves. Numbers, sets, and other mathematical objects would be prime candidates for the first sort of thing, while familiar physical objects like people and planets and stars would be examples of the second kind of thing” [10].

Philosopher Brian Davies also gives an explanation and ties it back in to the cosmological argument:

“a contingent matter — something which, though in fact the case, does not, absolutely speaking, have to be the case. And with that thought in mind you might then go on to suggest that reasons are called for when something that does not have to be the case (a contingent fact) actually turns out to be the case. Some statements look as though they tell us something that could not possibly be false. Take, for example, ‘Any triangle has three sides’ or ‘Any bachelor is unmarried.’ These statements do not report truths for which external reasons need to be invoked. As philosophers would commonly say, they are necessarily true. But is your holding this book now true of necessity? If not, you might suppose that there are reasons which account for why you are doing so. You might argue that, when it comes to contingent truths or facts, reasons outside them should always be sought. And that, we may say, is the ‘big idea’ informing the cosmological argument from sufficient reason. Is ‘The world exists’ contingently true? Or is it true of necessity?” [11].

Please note that I am not here defending or offering assent to these Leibnizian contingency arguments. I think they’re interesting and worth consideration, but that will have to be the object of a later study. For now, I discuss them for two reasons. The first is that these arguments, though not as popular as the kalam, are far more common than Thomistic arguments. It is likely that in any discussion of an “argument from contingency,” some might automatically assume I am referring to the Leibnizian version, when I’m not. Likewise, some assumed that my posts on Aquinas’s First Cause argument were actually about the kalam, and so I had to carefully distinguish the two. The second reason is that, though they are completely different arguments, they employ similar terms/concepts, and these need to be understood properly. So what does Aquinas mean by contingency?

In contemporary philosophy, as we’ve seen, something is contingent if it could have possibly been otherwise than it was. Obviously, if something could have been otherwise, then there must be a reason why it is the way it actually is, and not any other way that it could possibly have been.

In a more technical sense, contemporary philosophers often think of a contingent being as a being which exists in at least one possible world but not all possible worlds, and a necessary being as a being which does exist in all possible worlds.

For example, in Craig’s defense of the contingency of the universe he writes:

“A possible world in which no concrete objects exist certainly seems conceivable . . . It’s easy to conceive of the non-existence of any and all the objects we observe in the world; indeed, prior to a certain point in the past, when the universe was very dense and very hot, none of them did exist” [12].

For those who aren’t familiar with the contemporary concept of “possible worlds”, that’s irrelevant for now; Thomists reject this anyways.

Thomistic philosopher Edward Feser writes:

“many [contemporary philosophers] think (for example) of what is necessary as that which exists in every possible world and of what is contingent as that which exists only in some possible worlds, or who assume that the notion of a necessary being must be that of a being the denial of the existence of which would entail a self-contradiction . . . Aquinas (like other Aristotelian essentialists) would not accept such modern accounts of necessity and contingency” [13].

We will discuss in the next post how exactly the two conceptions are different. For now, we will just say that contemporary use of “contingent” refers to the logical possibility and necessity of an object; whereas Aquinas refers to actual possibility and necessity. For Aquinas, a being is contingent if it has a tendency to come into or go out of existence over time, and necessary if it does not.

In conclusion to this introduction, I’d like to look again at the four “problems” which cosmological arguments must answer, as the quote from Alexander Pruss says above, and how the Thomistic arguments go about answering them.

The first problem Pruss labels the “Glendower Problem,” which is that “although some features, such as the existence of contingent things, call for an explanation, it can be disputed whether an explanation actually exists” [14]. Aquinas’s answer to this is his metaphysical causal principle, specifically of efficient causation, which is discussed in some detail in the articles of my two previous series. The Third Way also makes use of the causal principle, but in a way that is unique to the other two Thomisitic cosmological arguments.

The second problem is the Regress Problem. As those who have read the previous series will be well aware, Aquinas bases his arguments entirely on examples of essentially ordered causal series, for which an infinite or circular regress is in principle impossible, thus avoiding the Regress Problem. This is the same in all of the Thomistic cosmological arguments.

The third Pruss calls the “Taxicab Problem,” which is, again, the “what caused God” question [15]. The actual problem is that in answering the Glendower Problem, we apply a causal principle to existing things; but then we must ask, does this same causal principle apply to that which is posited as the causal explanation of the specific feature of the universe in question, and if so, in what way? Consider this horrendous caricature of a cosmological argument as an example:

  1. Everything that exists has a cause
  2. The universe exists
  3. Therefore the universe has a cause

Here, the causal principle used is that everything must has a cause. But then the cause of the universe, which itself exists, must also have a cause. Most atheists who poke fun at this type of argument conclude from its ridiculousness that the universe itself must not require a cause, because if it did, we would end up in an infinite regress. But that is not actually the problem. Many atheist philosophers, following Hume, don’t have a problem with infinite regresses. The argument given above is a logically valid argument; if its premises are true, the conclusion is guaranteed. Its ridiculousness is not (necessarily) that it ends in an infinite regress, because the series in question would presumably be an accidentally ordered series. So quipping that “if everything has a cause, then the cause of the universe must have a cause, and therefore we’d end up in an infinite regression of causes” does not actually defeat the argument. Rather the problem lies with premise one, which most atheist and theistic philosophers alike would reject.

Aquinas’s solution to the Taxicab Problem lies in the very nature of his solution to the Glendower Problem. The principle of efficient causation does not state that everything has a cause, but only that the actualization of potency requires a cause; and since God is Pure Act, there is nothing in principle which could “cause” God. This, again, is true of all three Thomistic cosmological arguments.

The final problem is the Gap Problem: once we have established a First Cause, why think the First Cause is God? I’ve dealt with this in the previous series and will be doing so further later in this series. Many people accuse Aquinas, in his presentation of the Five Ways, of failing to solve the Gap Problem; but this, also, is untrue. Aquinas spends quite a large amount of space showing why his First Cause is a being very much like our conception of God.

So then, what is the actual difference between the three Thomistic cosmological arguments? The major difference comes from their stating places. The First Way starts from motion; the Second Way starts from efficient causation in general; and the Third Way starts from the existence of contingent things.

In the next post, we will begin to lay out the argument itself.





[1]. Craig, William Lane. The Cosmological Argument From Plato to Leibniz. Eugene: Wipf and Stock Publishers, 2001. ebook, x.

[2]. Moreland, J. P. and Craig, William Lane. Philosophical Foundations for a Christian Worldview. Downer’s Grove: InterVarsity Press, 2003. Print, 466.

[3]. Pruss, Alexander R. “The Leibnizian Cosmological Argument.” The Blackwell Companion to Natural Theology, edited by William Lane Craig and J. P. Moreland, Blackwell Publishing, 2012, 24.

[4]. Ibid.

[5]. Ibid.

[6]. Ibid., 25.

[7]. Ibid., 25-26.

[8]. See Craig, William Lane. Reasonable Faith: Christian Truth and Apologetics. 3rd ed. Wheaton: Crossway, 2008. Print, 106.

[9]. Davidson, Matthew, “God and Other Necessary Beings”, The Stanford Encyclopedia of Philosophy (Spring 2015 Edition), Edward N. Zalta (ed.), URL = <>.

[10]. Craig, William Lane. Reasonable Faith: Christian Truth and Apologetics. 3rd ed. Wheaton: Crossway, 2008. Print, 107.

[11]. Davies, Brian. An Introduction to the Philosophy of Religion. 3rd ed. New York: Oxford University Press, 2004. Print, 55.

[12]. Craig, William Lane. Reasonable Faith: Christian Truth and Apologetics. 3rd ed. Wheaton: Crossway, 2008. Print, 109.

[13]. Feser, Edward. Aquinas. London: Oneworld Publications, 2009. Print, 91.

[14]. Pruss, Alexander R. “The Leibnizian Cosmological Argument.” The Blackwell Companion to Natural Theology, edited by William Lane Craig and J. P. Moreland, Blackwell Publishing, 2012, 24.

[15]. Ibid.


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