What Are the Classical Proofs for God’s Existence?

Before the onset of Christianity, and subsequently throughout Christian history, a number of proofs have arisen regarding the existence of God. To be classical means that an argument utilizes philosophical categories and concepts found in the philosophy of the ancient Greeks and improved upon by Christian philosophers.[1] The classical proofs stand in contrast to the more popular, contemporary arguments for God’s existence.[2] Further, because philosophy in general, and Greek philosophy in particular, is relatively unknown by the average person, the popular, contemporary arguments (culling as they do from the more familiar categories and data of modern science) generally resonate with people more so than do the classical proofs.[3]

Classical Proofs

Broadly speaking, both contemporary and classical arguments for God’s existence fall within three categories:

(1) arguments for God as the cause of the existence of the universe (cosmological arguments),

(2) arguments for God as the cause of the design or purpose of the universe (teleological arguments), and

(3) arguments for God as the cause of human morality (moral arguments). Let’s review them in order.

Cosmological Arguments

There are different versions of the cosmological argument. One of the most important differences between the classical cosmological argument and the contemporary arguments is that the contemporary arguments aim to show that God is the cause of the universe coming into existence a finite time ago.[4] In contrast, the argument from the medieval philosopher Thomas Aquinas (c. 1225–1274) demonstrates that God is the cause of the current existing of the universe.[5]

The Kalam Cosmological Argument. The kalam cosmological argument appeals to the latest findings in science to show that the universe had a beginning a finite time ago in the big bang.[6] The argument says:

(1) the universe began to exist;

(2) whatever begins to exist must have a cause;

(3) therefore, the universe has a cause.

In defense of the first premise, contemporary scientific data are marshalled regarding the big bang theory, the expansion of the universe, and the second law of thermodynamics. In the big bang theory, scientists maintain that the universe began in a colossal explosion a finite time ago. The significance of this is that since the

universe has not existed from eternity, it must have come into existence in the finite past. The expansion of the universe says that every object in the universe is moving away from every other object such that even space itself is expanding. The significance of this is that the universe could not have been expanding from eternity; otherwise, it would be infinitely dispersed (which it is not). Therefore, the universe came into existence a finite time ago. The second law of thermodynamics says that ‘all isolated systems will tend toward a state of maximum disorder (entropy)’. In an isolated system, the amount of energy available to do work decreases and becomes uniform.

This amounts to saying that the universe is “running down” (much like the batteries of a flashlight left on for an extended period of time). The significance of this is that the universe could not have been running down from eternity; otherwise, it would have run down by now—which it has not.

Therefore, the universe came into existence a finite time ago. Since the universe came into existence and because whatever comes into existence must have a cause, based on the law of causality, then the

universe must have had a cause. Since this cause created matter, it must be immaterial. Since this cause created time, it must be timeless. Since this cause created space, it must be spaceless. For if any of these finite conditions (space, time, and matter) were part of the cause, it would be tantamount to saying the cause caused itself to be, which is absurd, for this would require the cause to exist prior to causing its own existence.

Since this cause created the universe, it must be of unimaginable power. Because the effect of this cause (the universe) has not existed forever with the cause, this cause must have willed it to exist, which means it is personal. Thus, we have an immaterial, timeless, spaceless, personal cause of unimaginable power. Many people recognize this cause as God.

A Cosmological Argument of Thomas Aquinas. Many people probably would not think in terms of something needing a cause of its current existing. Consider this as an analogy of how Aquinas understood existence (or existing).[7] Suppose you saw a giant ten-foot glass ball in front of a local business. You might ask where it came from. If you were told that it was a promotional tool celebrating the grand opening of the business and that the glass ball had been manufactured at a local glass factory, you no doubt would find this explanation satisfactory.

Now suppose that you began to hear music playing. Most likely you would not ask (as you did about the glass ball) where the music came from. Instead, you would ask where the music is coming from. This is because you realize that music exists as music only as long as it is being caused to be music. As soon as the cause of the music stops causing the music, the music ceases to exist. For Aquinas, the existence of all created, finite things was like the music. Existence (or existing) is an act. It is something that essences do.[8] You can find in his (and others’) writings the expression “the act of existence.”

How does this notion of existence fit into an argument for God’s existence? Consider yourself as a human being. Your essence (or nature) is what makes you a human. Your existence is what makes you a being. Now, whatever is true of you is true of you either by virtue of your essence or not.

For example, the fact that you have rationality is because you are a human. It is part of your essence as a human to have rationality. But consider the fact that you are reading this chapter. Is the reason you are reading this because you are a human? Is it part of your essence as a human to be reading this book? The answer is no; otherwise, those who are not reading this article would  not be human. However, you can easily account for why you are reading this chapter even though it is not part of your essence to do so. You are reading this chapter because you are causing yourself to read this book.

Now consider the fact that you exist. Is the reason you exist because you are a human? Is it part of your essence to exist? The answer, again, is no; otherwise, you would have always existed.[9] If not, then what is causing your current existing (like the current existing of the music)? You cannot be the cause of your own existing, or you would then have to exist (as a cause) before you exist, which is impossible. This ultimately leads to the conclusion that the only way to account for your existing is that you are being caused to exist by something whose very essence is existence itself. As Aquinas remarked in another context: “All men know this to be God.”[10]

Teleological Arguments

The term teleological comes from the Greek word telos, meaning “end” or “goal” or “purpose.” In contemporary apologetics, the term more often used is design. However, the classical teleological argument differs markedly from the contemporary design arguments (see the chapter “What Is the Scientific Case for Intelligent Design?”). In these design arguments, the appeal is made to various scientific aspects of the universe, such as finetuning, [11]irreducible complexity,[12] and information theory[13] to argue for an intelligent designer of the universe.[14]

In the classical teleological argument, given in the fifth of Aquinas’s famous “Five Ways” (his five arguments for God’s existence in his Summa Theologiae), the focus is not on these physical aspects of biological life forms. Instead, it is on the metaphysical aspects of such living beings, particularly final causality.[15] What does all this mean? In classical thought,

there are four kinds of causes.[16] Consider the example of making a chair out of wood. The material cause is that out of which an effect is—i.e., what it is made of, namely, wood. The formal cause is that which an effect is—i.e., its form, structure, or nature, such as chair-ness. The efficient cause is that by which an effect is—i.e., who or what produced the chair, namely, the carpenter. The final cause of the chair is that for which an effect is—i.e., why it was built, such as to sit on.

The teleological argument says that because all things aim toward their destiny (acorns grow into oak trees, zygotes grow into adult humans), unless hampered by an external impediment, there must be someone who is directing things to their telos. Why? Thomist philosophers (philosophers who follow the philosophy of Thomas Aquinas, who followed, for the most part, the philosophy of Aristotle) offer several reasons. First, the end goal (e.g., the oak tree for the acorn) is the cause (i.e., the final cause) of the acorn moving toward becoming a full-grown oak tree. But something cannot move invariably toward its destiny if it does not have a mind, because aiming toward a purpose can only be done by intelligence. This intelligence we call God.[17]

Second, how can the full-grown oak tree be a cause of anything if it does not actually exist in the acorn? To be sure, it exists potentially, but a thing that exists only potentially cannot itself be a cause. If the final cause does not actually exist in the acorn, the only place it can exist is in an intellect. The explanation is that God, having the purpose or goal or telos of the acorn “in His mind,” so to speak, moves all things toward their appropriate destiny analogous to how the house “exists” in the intellect of the architect/carpenter who builds it. The “final cause” of the oak tree is in the mind of God as its Maker.

Moral Arguments

As with both the cosmological and teleological arguments, the moral argument in the classical tradition differs in important ways from the popular, contemporary moral argument, which says that if God does not exist, then objective moral values do not exist. But objective moral values do exist. Therefore, God exists.[18]

In the classical tradition, the issue of human morality is called natural law theory and is a subset of the broader issue of law, both of which are nested in a context of specific philosophical notions regarding natures (or essences), teleology, goodness as such, moral goodness in particular, the relationship of being and goodness and more.[19]

Natural law theory focuses on how it is that human beings, unique among God’s creatures on earth (having as we do rationality and free will), intersect with God’s superintendence of all of His creation. Minerals, plants, and animals conduct themselves according to the laws of the physical world (minerals, plants, and animals), with a modicum of life that requires the intake of nutrients for growth (plants and animals), and with sensory faculties like seeing or hearing (animals).

God sovereignly manages the affairs of all of His creation. He aims His creation toward its proper goal or end. Physical forces and elements always behave the way God has created them to behave. Plant seeds always grow toward the specific kind of plant they are unless impeded by some outside force. Animals always grow and behave the way God has created them to.

They all obey God’s will without knowing they are doing so. God has created them to be and do in accordance with their natures. They never fail to do so. This end or goal toward which all things aim is called good.[20]

Human beings are unique among God’s sensible creatures. Unlike minerals, plants, and animals, humans have rationality and free will. We are able to choose in accordance with or in violation of our natures. Just as God has created the acorn such that, because of its nature, it will grow up to be an oak tree, God has created humans with a nature that aims us at being the kind of creatures God intends us to be. Being created in His image, God intends for us to be holy as He is holy (Matthew 5:48; Romans 6:19; 2 Corinthians 7:1; Hebrews 12:10). But unlike the acorn, humans can freely choose to disobey God’s will. Doing so is called sin. Sin does violence to what God intents as our good (i.e., our telos). The capacity to choose for or against God’s good for us is what makes our actions moral. This is why no action of an animal is a moral action.

A Classical Moral Argument. How does this give us a classical moral argument for God’s existence? Notice what the classical approach does not argue. The popular, contemporary moral argument says that God is necessary for morality’s objectivity. But in the classical tradition, morality’s objectivity arises initially from the fact that the moral good for a human being is to be the kind of being a human ought to be by virtue of human nature. The good of any being is to gain the perfections (i.e., to progress toward its end or telos) it ought to have by virtue of its nature.[21] A huma ought to have certain perfections because of what it means to be a human.

Thus, it is not the objectivity of morality that is in question. What is in question is how the elements constituting human morality point to God as the ultimate explanation for human morality.

But can we know what we ought to be by virtue of our human nature? There are two sources. First, there is a general source based upon God’s general revelation. That is, God has revealed truths through His creation that are knowable by reason (as opposed to faith).[22] Second, God has also spoken through His special revelation—what we know as the Bible. So, while natural law can tell us that murder is wrong, Jesus expanded upon this (external) moral truth to tell us that even being angry (internal) without a cause is also wrong (Matthew 5:22). While the data of special revelation can very well convict the unbeliever (Romans 10:17), as a general rule, no human being (saved or lost) misses out on the data of general revelation.

The classical moral argument, then, can take that which is known about morality and demonstrate what must be true about reality to account for morality. Having identified this, those same aspects can be “rearranged” to formulate a classical cosmological argument like the one laid out above.

Human morality concerns itself with human teleology, which is to say, human good. But what is good for a human arises from the fact that human beings have natures or essences. This also points to final causality and efficient causality. Human nature (or essence) can be distinguished from a given human’s existence. But anything that has existence, yet whose existence is not because of its nature or essence, can only have existence because it is being given existence (i.e., caused) by something whose essence is existence itself. And all people know this to be God.

A Witness That Always Speaks to Us

The existence of God is crucial to the Christian apologetic. For if God does not exist, there cannot be a Son of God (Jesus), acts of God (miracles), salvation offered by God, or the Word of God (Bible). To be certain, God gave us a general witness of Himself in creation, and a particular witness in the nature (or essence) of reality. This witness is both intelligible and accessible, always speaking to us of the eternal God who is.

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[1] Colin Brown, Philosophy and the Christian Faith (Downers Grove, IL: InterVarsity, 1968), 250; Terry L. Miethe, ed., I Am Put Here for the Defense of the Gospel: Dr. Norman L. Geisler: A Festschrift in His Honor (Eugene, OR: Pickwick, 2016), 233-256. Regarding the legitimacy and value of utilizing philosophy in arguing for God’s existence: While there certainly have been (and continue to be) those who think that the Christian faith ought not utilize the tools, methods, categories, and claims arising from philosophy, there have been (and continue to be) those who recognize the proper role that philosophy can play in Christian theology in general and Christian apologetics in particular.

[2] Frank Turek, Stealing from God: Why Atheists Need God to Make Their Case (Colorado Springs, CO: NavPress, 2014) and J. Warner Wallace, God’s Crime Scene: A Cold-Case Detective Examines the Evidence for a Divinely Created Universe (Colorado Springs, CO: David C. Cook, 2015).

[3] Joseph Owens in contrasting the classical proofs with the popular, contemporary arguments when he said, “Other arguments may vividly suggest the existence of God, press it home eloquently to human consideration, and for most people provide much greater spiritual and religious aid than difficult metaphysical demonstrations. But on the philosophical level these arguments are open to rebuttal and refutation, for they are not philosophically cogent.” Joseph Owens, “Aquinas and the Five Ways,” Monist 58 (Jan 1974), 33.

[4] This version is known as the Kalam cosmological argument. It arose prominently in the Middle Ages. It was revived in contemporary thinking largely due to the work of Christian philosopher and apologist William Lane Craig in The Kalam Cosmological Argument (London, UK: Macmillan, 1979; republished Eugene, OR: Wipf & Stock, 2000). According to Craig, the term kalam literally means “speech” in Arabic. For a defense of the argument against objections raised after Craig published his work, see Richard G. Howe, An Analysis of William Lane Craig’s Kalam Cosmological Argument, unpublished master’s thesis (Oxford, MS: University of Mississippi, 1990). For a more succinct treatment and defense of the argument, see J.P. Moreland, Scaling the Secular City: A Defense of Christianity (Grand Rapids, MI: Baker, 1987), 18-33.

[5] It is interesting to note that Aquinas’s argument is indifferent as to whether the universe came into existence a finite time ago or if it has existed from all eternity. As a Christian, Aquinas certainly believed that the universe began a finite time ago in creation, just as Genesis says. But the nature of his argument does not play off of this aspect of the universe.

[6] In its medieval version and in its contemporary academic version, the Kalam cosmological argument involves the notion of the impossibility of infinite temporal regression. Appealing to the mathematical nature of infinite sets, it argues that the past cannot be infinitely long; therefore, the universe must have had a beginning. If it had a beginning, then it must have had a cause. This cause is God.

[7] For philosophical treatments (primary and secondary) of the notion of being (existence) in Aquinas, see Thomas Aquinas, On Being and Essence, trans. Armand Maurer, 2d rev. ed. (Toronto, Canada: Pontifical Institute of Mediaeval Studies, 1983); Commentary on Aristotle’s Metaphysics, trans. John P. Rowan (Notre Dame, IN: Dumb Ox Books, 1961); Dominic Báñez, The Primacy of Existence in Thomas Aquinas, trans. Benjamin S. Llamzon (Chicago, IL: Henry Regnery, 1966); Étienne Gilson, Being and Some Philosophers (Toronto, Canada: Pontifical Institute of Mediaeval Studies, 1952); and Joseph Owens, An Interpretation of Existence (Houston, TX: Center for Thomistic Studies, 1968). For an analytic philosophical perspective on existence that stands in contrast to Aquinas’s, see William Lane Craig and J.P. Moreland, Philosophical Foundations for a Christian Worldview (Downers Grove, IL: InterVarsity, 2003), 187-193. For a broader exploration of the notion of existence in ancient and medieval philosophy, see Parviz Morewedge, ed. Philosophies of Existence: Ancient and Medieval (New York: Fordham University Press, 1982).

[8] A thing’s essence is that aspect of the thing by virtue of which it is the kind of thing it is. Thus, a human being is a human being precisely because it possesses a human essence. A dog is a dog precisely because it possesses a dog essence. Another word for essence is nature.

[9] What is more, there are things that have the essence of human but do not have existence, such as Sherlock Holmes or Aragorn. They are conceptual beings or beings of reason, otherwise known as fictional beings. But if it was of the essence of humans to have existence, then Sherlock Holmes and Aragorn could not fail to really exist.

[10] Thomas Aquinas, St. Thomas Aquinas Summa Theologica: Complete English Edition in Five Volumes, trans. Fathers of the English Dominican Province (Westminster, MD: Christian Classics, 1981), I, Q2. What is an added strength of the classical argument over the popular, contemporary arguments is that in exploring more of what is entailed by seeing that God’s essence is existence itself, one discovers that this God has all the superlative attributes of classical theism. For a treatment of these classical attributes, see James E. Dolezal, All That Is in God: Evangelical Theology and the Challenge of Classical Christian Theism (Grand Rapids, MI: Reformation Heritage, 2017).

[11] Philosopher Robin Collins describes the fine-tuning argument this way: “When scientists talk about the fine-tuning of the universe they’re generally referring to the extraordinary balancing of the fundamental laws and parameters of physics and the initial conditions of the universe. Our minds can’t comprehend the precision of some of them. The result is a universe that has just the right conditions to sustain life. The coincidences are simply too amazing to have been the result of happenstance.” See Robin Collins, “The Evidence of Physics: The Cosmos on a Razor’s Edge” in Lee Strobel, The Case for a Creator: A Journalist Investigates Scientific Evidence That Points Toward God (Grand Rapids, MI: Zondervan, 2004), 130.

[12] Irreducible complexity refers to the fact that there are systems in biological life that consist of several interlocking parts that must be in place before they can function at all. The argument demonstrates that this irreducibly complex system could not have come about by gradual, incremental changes, but must have been given all at once. Even Charles Darwin himself admitted, “If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.” See Charles Darwin, Origin of Species, chapter 6, “Difficulties of the Theory” section “Modes of Transition” in Robert Maynard Hutchins, ed. in chief, Great Books of the Western World, vol. 49 (Chicago, IL: Encyclopedia Britannica, Inc.), 87. The definitive work on irreducible complexity is Michael J. Behe, Darwin’s Black Box: The Biochemical Challenge to Evolution (New York: The Free Press, 1996).

[13] Information theory, sometimes referred to as intelligent design, refers to the fact that the DNA of biological life forms contains information encoded at the molecular level. Because information always arises from intelligence, there must be an intelligent source of the DNA’s information. Mechanical engineer Walter Bradley and biochemist Charles Thaxton explain, “Proponents of an intelligent origin of life note that molecular biology has uncovered an analogy between DNA and language…The genetic code functions exactly like a language code— indeed it is a code. It is a molecular communications system: a sequence of chemical ‘letters’ stores and transmits the communication in each living cell.” See Walter L. Bradley and Charles B. Thaxton, “Information and the Origin of Life,” in J.P. Moreland, ed., The Creation Hypothesis: Scientific Evidence for an Intelligent Designer (Downers Grove, IL: InterVarsity Press, 1994), 205.

[14] Most contemporary design arguments follow in the tradition of William Paley (1743–1805) and his famous watchmaker argument from his work Natural Theology: or Evidences of the Existence and Attributes of the Deity, Collected from the Appearances of Nature (Philadelphia, PA: John Morgan, 1802).

[15] Edward Feser describes the difference this way: “One key difference between the design argument and the Fifth Way…is that whereas the former takes for granted a ‘mechanical’ conception of the natural world…Aquinas’s argument crucially presupposes that final causes are as real and objective a feature of the natural world as gravity or electro-magnetism.” See Edward Feser, Aquinas: A Beginner’s Guide (Oxford, UK: Oneword, 2010), 112.

[16] The classical understanding of the four causes is from Aristotle. He discusses the four causes in Metaphysics D (V), 2, 1013a24-1013b3.

[17] Aquinas says it this way: “We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.” See Aquinas, Summa Theologica, I, Q2, Art. 3.

[18] Examples of this kind of moral argument for God’s existence include William

Lane Craig, Reasonable Faith: Christian Truth and Apologetics, 3d ed.

(Wheaton, IL: Crossway, 2008), 172-183; and Chad V. Meister, Building Belief: Constructing Faith from the Ground Up (Grand Rapids, MI: Baker, 2006), 110- 121. Among other things, the argument aims to show that without a transcendent, objective moral standard for right and wrong, there is no way to account for that which everyone knows to be the case—that such objective moral values do exist. It is challenging to find this argument in the classical tradition. This is not to say that the classical tradition sees no connection between morality and God. Rather, that connection is somewhat more complex and hinges.

[19] An excellent reading on being and goodness is Jan A. Aertsen, “The Convertibility of Being and Good in St. Thomas Aquinas,” New Scholasticism 59 (1985), 449-470.

[20] The “good” of a plant seed is its growing toward the perfection of being the kind of plant God intends it to be. The “good” for a newborn puppy is its growing toward the perfection of being the kind of dog God intends for it to be. To be sure, this “good” is not a moral good. Instead, it is good as such. Good means the end goal toward which a thing’s potentials aim in accordance to its nature, which is to say, in accordance to the kind of thing God has created it to be. When humans study these aspects of God’s creation, this gives rise to what we call the sciences (e.g., physics, geology, biology, etc.). These laws or regularities can never be violated except by miraculous intervention from the Creator.

[21] By analogy, a knife “ought” to have a sharp blade because that is what it means to be a knife. It would not make any sense for someone to object by saying, “Who are you to say that a knife ought to have a sharp blade!”

[22] This is why virtually all the world’s religions and philosophies have the same general views about how humans ought to act—obligations such as do not kill another human, respect others’ property, keep your promises, nurture the children, respect the elders, and more are found throughout all times and places. There are, of course, exceptions. There are also instances when, even knowing what we ought to do, we still fall short. Yet the history of ideas shows a remarkable pattern to human behavior. Romans 2:14-15 tells us, “When Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them.” See C.S. Lewis, Mere Christianity (New York: Macmillan, 1960) and The Abolition of Man (New York: Macmillan, 1955).