January 1, 2001

The Cosmological Argument in Plato and Aristotle

Historically, the cosmological argument has been one of the most common arguments that philosophers have used to prove the existence of God. (The argument, as philosopher William Lane Craig writes, “assumes that something exists and argues from the existence of that thing to the existence of a First Cause or a Sufficient Reason of the cosmos” [Craig, Reasonable Faith 79]). Although this proof has primarily been used to prove the existence of the Christian God—being used by such people as Anselm, Aquinas, Scotus, Descartes, Leibniz, and Berkely—it has also been used by such non-Christians as Plato, Aristotle, al-Farabi, Spinoza, and Locke. In the following paper, I explicate the versions of the argument used by both Plato and Aristotle. Since the argument has primarily been used to prove the existence of the Christian God, in the final section of the paper, I briefly compare the god, or gods, that Plato’s and Aristotle’s arguments point to with the God of Christianity.

The cosmological argument in Plato’s writings
Plato’s cosmological argument can be found in the tenth chapter of the Laws, in an exchange between Clinias and an Athenian stranger. The Athenian begins his attempt to prove the existence of the gods by arguing that, of all the different types of motion, the motion “which can move itself” is “necessarily the earliest and mightiest of all changes” (895b).
When we have one thing making a change in a second, the second, in turn, in a third, and so on—will there ever, in such a series, be a first source of change? Why, how can what is set moving by something other than itself ever be the first of the causes of alteration? The thing is an impossibility. But when something which has set itself moving alters a second thing, this second thing still a third, and the motion is thus passed on in course to thousands and tens of thousands of things, will there be any starting point for the whole movement of all, other than the change in the movements which initiated itself? (894e-895a)
Implicit in his argument is the belief that an infinite regress cannot occur. If an infinite regress cannot occur, then there must have been a first motion. It then follows that this first motion must have set itself into motion—for there would have been nothing else to set it into motion. W.T. Jones summarizes this argument nicely: “if we want to avoid an infinite regress of causes of causes of causes . . . we must sooner or later admit the existence of a self-caused motion” (Jones 196).

After showing that the motion “which can move itself” must have been the first motion, the Athenian argues that that motion must be alive; for, when a “thing moves itself, we speak of it as alive.” He then points out that people generally regard something as being alive if it has a soul (psyche). Since this is the case, it follows that soul—which, we should notice, Plato regards as an immaterial being, or principle (Koons)—is the “motion which can set itself moving” and is, therefore, the cause of other motions (“source of movement”). The Athenian then argues that, since soul is the cause of other motions, it must govern all things. Among other things, this means that soul governs physical objects, including “heaven itself” (895c-896e).

Next, the Athenian shows that there must be at least two souls causing motion—one causing regular, or “beneficent” motion and another causing irregular, or bad motion (896e). Regular motion, he believes, is good since it bears a resemblance to wisdom; while irregular motion is bad since it resembles folly. Since the motion of the heavenly bodies is regular, or wise, and, therefore, good, the Athenian reasons that the soul, or souls, that moves the heavenly bodies must also be wise and, therefore, good. Because these souls are good and are the “causes of all,” the Athenian concludes that they must be gods (theoi) (899b).

Aside from what we have already learned about the gods—they are alive, immaterial, in control of all things, wise, and good—Plato’s cosmological argument does not tell us much more about them. However, we should note one more significant thing that this argument tells us about the gods—they are interested in the affairs of humans. As the Athenian reasons, since the gods are good and since one who is negligent in either big, or small matters cannot be considered good, the gods cannot negligent and, therefore, must care at least somewhat about the affairs of men (899d-903e).

The cosmological argument in Aristotle’s writings
Aristotle’s cosmological argument can be found both in the Physics and Metaphyics. (Since his most thorough proof for the existence of god is found in the Physics, we will begin there. When we start to consider what type of being he believes god to be, we will also look at his comments in the Metaphysics.) Aristotle’s proof for god’s existence can be broken down into three parts: (1) all things that are in motion must be moved by something, (2) that mover which is ultimately responsible for moving another thing must be responsible for its own motion, and (3) that which moves itself cannot be completely in motion.

Beginning in Book V of the Physics, Aristotle claims that “all things that are in motion must be moved by something” (256a3). As Craig writes, in order to understand why Aristotle believes this to be true, we must understand his “actuality/potency distinction.” Craig points out that Aristotle believes that all change, motion being a type of change, involves the “actualizing of some potency. It is because things have real potencies that they are able to change.” Therefore, the “implicit assumption” in this claim “is that the potential cannot actualize itself. The potential, precisely because it is potential, cannot make itself actual” (The Cosmological Argument 25).

Next, Aristotle claims that the mover that is ultimately responsible for moving another thing must be responsible for its own motion (whether it directly moves it, or begins a series of movements that eventually move the final object). Although he uses a number of arguments to prove this claim, his first argument seems to be the easiest to follow (256a10-20). In the argument, Aristotle essentially restates Plato’s argument that, since an infinite regresses cannot occur, the ultimate mover in a series must be self-moved. To help make this point, he gives an example of a man who uses a stick to move a stone. In this example, it is clear that the stick, which is not ultimately responsible for moving the rock, is not self-moved—it is moved by a hand. Similarly, the hand, which also is not ultimately responsible for moving the rock, is not self-moved—it is moved by a person. But the person, who is the ultimate cause of the rock’s movement, is self-moved. All series of movements, Aristotle believed, must be like this—the ultimate mover in the series must be responsible for its own motion.

Finally, after showing that everything in motion is moved by something else and that the ultimate mover must be self-moved, Aristotle claims that that which moves itself cannot “in its entirety move itself” (257b2). If all of a self-mover were movable, then it would be, at the same time, both a potentiality and an actuality—for such an object would, at the same time, be both in motion and causing motion. However, since it is absurd to say that an object can be both a potentiality and actuality at the same time, Aristotle reasons that a self-moving object cannot be entirely in motion. “Therefore when a thing moves itself it is one part of it that is the movent and another part that is moved” (257b2-10). Aristotle further points out that “each of the two parts” of a self-moving object cannot be moving one another. If such were the case, then there would be “no first movent.” However, as he showed earlier, every moving object must have a first mover (257b10-20).

Now that we have seen Aristotle’s proof for the existence of this unmoved mover, which he refers to as god (theos), we should find out what this mover is like. First, there is only one unmoved mover. One of Aristotle’s reasons for believing that there can only be one mover is his belief that, since positing more than one mover is unnecessary, it should, therefore, be avoided. “We ought . . . to suppose that there is one rather than many [first movers], and a finite rather than an infinite number. When the consequences of either assumption are the same, we should always assume that things are finite rather than infinite in number” (259a5-10). Aristotle also believes that this mover must be one because of the continuity of motion. Since motion has always existed (his proof for the eternality of motion can be found in Book VIII, Section 1., of the Physics), he believes that it must be continuous—“what is always is continuous, whereas what is merely in succession is not continuous.” If an object’s motion is continuous, then it is one. And it can only be one if it has just one mover; for, if a thing in motion had more than one mover, then it would be moved “now by one thing and now by another” and “the whole motion will not be continuous but successive” (259a15-20).

Second, this unmoved mover must be pure actuality.
[If] there is something which is capable of moving things or acting on them, but is not actually doing so, there will not be movement; for that which has a capacity need not exercise it . . . that which is potentially may possibly not be. There must, then, be such a principle, whose very substance is actuality.” (Metaphysics 1071b12-20).
Hidden in this argument is the assumption that eternal motion necessarily exists. Since eternal motion necessarily exists, its cause must be actuality—for if it were potentiality, then it might not exist (Craig, The Cosmological Argument 35). Third, this mover must be eternal. Aristotle’s simplest argument for the unmoved mover’s eternality is that, since motion is eternal, “the first movent . . . will be eternal also” (Physics 259a5-10). Fourth, this mover is indestructible. Aristotle believes that the mover must be indestructible because eternal motion is indestructible. Since eternal motion is indestructible and since the unmoved mover caused this motion, the mover must also be indestructible (Metaphysics 1071b7-9) (Owens 438-39). Fifth, this mover must be immaterial. As Aristotle writes, “these substances must be without matter; for they must be eternal, at least if anything else is eternal” (Metaphysics 1071b20). As Craig points out, although Aristotle’s reasoning here does not seem to follow, what he “probably means to assert is that because the substance is pure actuality . . . it must be incorporeal, for matter involves potentiality” (The Cosmological Argument 34).

Sixth, Aristotle writes that the unmoved mover must be a final cause—an object’s desire, or purpose (theos). He believes this to be so because it moves in the same way that objects of thought and desire move. As he writes, “. . . there is a mover which moves without being moved . . . And the object of desire and the object of thought move in this way; they move without being moved” (1072a24-27). The fact that Aristotle’s unmoved mover is a final cause has several implications. Since the mover is an object of desire, Aristotle reasons that it “produces motion by being loved” (1072b2)—in other words, that it moves objects by being loved by them. Since it is an object of desire, it also follows that the mover must be good. And since the mover is good, it must live the best life—“its life is such as the best which we enjoy” (1072b15). And since it lives the best life and since “the act of contemplation is what is most pleasant and best” (24), Aristotle concludes that it must life a life of contemplation.

We have now established that the mover lives a life of contemplation, or to put it another way, a life of thought, or reflection. So it is now only natural to ask what the mover thinks about. Aristotle believes that, since the mover is the best and must, therefore, think about that which is best, it follows that it must think about itself. And since the mover cannot change, “for change would be change for the worse,” it must continually think about itself. “Therefore it must be itself that thought thinks (since it is the most excellent of things), and its thinking is a thinking on thinking” (1074b15-35). As Jones points out, since this mover only thinks about itself, it follows that it cannot think about humans and, therefore, that it cannot be concerned with the affairs of humans (Jones 231).

Since we have now looked at the arguments of Plato and Aristotle, and have seen the types of gods that their proofs point to, we are prepared to compare their gods with the God of Christianity. The first thing we should notice is that, in many ways, the arguments of both Plato and Aristotle lead to gods far different than the Christian God. Plato’s argument, for example, points to many gods, not one. Further, as Jones writes, “Plato’s god is not really an object of worship; his god is good, but hardly a loving father” (202). Likewise, although Aristotle calls his unmoved mover god, this mover is, as Jones writes, simply a “metaphysical necessity—the system requires an unmoved mover, a completely actual and fully realized form, but he is not an object of worship” (231).

Although both Plato and Aristotle’s arguments point to gods, or principles, that are, in many ways, far different than the God of Christianity, it is worth noting that their gods have many similar traits to the Christian God. Plato’s gods, for example, are immaterial, in control of the cosmos, wise, good, and interested in human affairs. Likewise, Aristotle’s mover is one, eternal, indestructible, immaterial, good, and supremely intelligent. So, in conclusion, we can easily see how—although their arguments do not prove the Christian God—Plato and Aristotle paved the way for future Christian philosophers to use the same basic proof to argue for the existence of the God of the Bible.

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Works Cited

Aristotle. Metaphysics. Trans. W.D. Ross. Oxford: Clarendon Press, 1952.

---. Physics. Trans. R.K. Gaye. Oxford: Clarendon Press, 1966.

Craig, William Lane. The Cosmological Argument from Plato to Leibniz. London: MacMillan Press, 1980.

---. Reasonable Faith. Wheaton, IL: Crossway Books, 1994.

Jones, W.T. The Classical Mind. Fort Worth: Harcourt Brace Jovanovich College Publishers, 1980.

Koons, Robert. “Lecture #3: Philosophy 356, Western Theism, Spring 1998, University of Texas.” <http://www.leaderu.com/offices/koons/docs/lec3.html>. 24 November 2000.

Owens, Joseph. The Doctrine of Being in the Aristotelian Metaphysics. Toronto: Hunter Rose Company, 1978.

Plato. Laws. Trans. A.E. Taylor. Princeton: Princeton University Press, 1989.