How does the ontological argument support the existence of God?
The ontological argument for God's existence is a particular kind of philosophical argument which Christian theologians have developed for centuries as a way of showing why it is rational to believe in God's existence. It is usually listed alongside several other such arguments for God's existence, including the cosmological argument, the "kalam" argument, the teleological argument, the moral argument, and others as well. Together, these arguments help to form the basis of a great deal of contemporary Christian apologetics.
The ontological argument was first proposed by St. Anselm (1033—1109) as an (allegedly) irrefutable proof of God's existence. Most modern philosophers, however, reject the argument (at least, in its simplest version) as inherently fallacious. There are, however, several prominent philosophers (most notably, Alvin Plantinga) who actually regard the argument as sound. In this article, we briefly outline and attempt to motivate the basic idea of the argument, deferring a more thorough discussion of possible objections and responses to the literature.
We begin with the observation that God, if He exists (and it is possible that He exists), is the greatest possible being; in fact, He is greater than anything else we could conceive of. To use philosophical language, God is "maximally great," by definition. However, if God did not exist, He would no longer be maximally great, for a God who did exist would also be greater than a God who did not exist. Therefore, God must exist.
If you get the feeling that the argument is playing some kind of trick on you, you are not alone: Arthur Schopenhauer famously called this argument a "charming joke." However, when the argument is formulated carefully, it turns out to be a deductively valid argument which hinges only on the assumption that it is possible that God exists. Thus, if we can show that it is possible that God exists, the rest of the argument follows logically.
Now, when we say that it is possible that God exists, we don't just mean something like "maybe He does, maybe He doesn't—I'm not sure" (the philosophical term for this sense of the word "possible" is epistemic possibility). Rather, by saying that it is possible that God exists, we mean to say that there is nothing which renders His existence logically impossible (the philosophical term for this sense of "possible" is metaphysical possibility).
To illustrate these two kinds of possibility, suppose I ask you whether the statement "123x848=104304" is true (without using a calculator or pencil and paper). Unless you are pretty good at mental multiplication, your answer will probably be "I don't know; it might possibly be true." Of course, this only reflects a limited ability to do mental mathematics; according to the basic rules of arithmetic, the answer is either necessarily false or necessarily true. Put slightly differently, it may be epistemically possible that this statement is true, but it is either metaphysically necessary or metaphysically impossible that it is true. By contrast, the statement "There is no intelligent life beyond the earth" is both epistemically possible and metaphysically possible: epistemically, because so far, no intelligent life has been discovered beyond the earth, and metaphysically, because there is no logical law or principle which requires us to believe that intelligent life actually exists beyond the earth.
So then, we return to the ontological argument: is it metaphysically possible that God (i.e., a maximally great being) exists? Indeed, it is hard to see how it could not be, since there is nothing intrinsically incoherent in such a notion. Moreover, there are several Christian philosophers who actually believe that a good case can be made for the metaphysical possibility of God's existence. If it really is metaphysically possible that God exists, then the rest of the ontological argument and its conclusion follow, making this argument quite powerful when it is formulated carefully.
In short, the ontological argument does not conclusively prove the existence of God, but it may be helpful when presented carefully, especially for skeptics who are rather philosophically minded. Although the ontological argument, along with the other apologetic arguments for God's existence, are (for the most part) not directly referenced in Scripture, it can be a valuable way of initiating and sustaining a conversation with someone for the sake of leading them to Christ. In the end, we must always be "prepared to make a defense to anyone who asks you for a reason for the hope that is in you; yet do it with gentleness and respect" (1 Peter 3:15), and understanding how Christians have historically defended their faith can play a vital role in gently and respectfully leading someone to Christ.
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