The following is a transcript from and Australian TV mini-series, ôBrides of Christ,ö which is to my knowledge the only dramatization of the ontological argument in the mass media. The characters are a teaching nun and her pupils, the novitiates at her convent.
Teacher: St. Anselm proved the existence of God in the following way: God, he argued, is that than which no greater can be thought. Does anyone not follow that?
[Several novices raise their hands.]
Teacher: Any concept, no matter how great, allows us to infer an identical one that does not exist, and the second one is ipso facto the greater. Since God must be the greatest thing anyone can think of, He must exist or He wouldnÆt be God.
Novice 1: He wouldnÆt be anything if He didnÆt exist, would He, Sister?
Teacher: Are you trying to be funny?
Novice 1: No, Sister. I just didnÆt understand the argument.
Teacher: Imagine two apples, equally juicy, crisp to the bite, round and firm, and of a brilliant green. Identical in every respect except, one exists, and the other one doesnÆt. Which is the greater?
Novice 2: The one that exists.
Teacher: Same goes for God.
Novice 1: That doesnÆt follow.
Teacher: St. AnselmÆs thoughts have been good enough for the greatest minds in the Church for nearly 900 years.
Novice 1: Well, itÆs just a game with words. God isnÆt a Granny Smith.
Teacher: Is that so?
Novice 2: And youÆre automatically assuming that existence is better than non-existence.
Teacher: Would you rather not exist?
Novice 2: You mean right now, or normally?
Teacher: If you want to stay in the Church, youÆll have to accept its teachings without question, as I do. Novice 2: But what if the Church is wrong?
Teacher: What sort of fool would that make me?
Novice 2: IÆm sorry, I didnÆt mean to question you.
AnselmÆs argument for the existence of God is known as the "ontological" argument, where 'ontological' means 'of or pertaining to being.' The basic idea of the argument is that the very being, nature, or essence of God implies that God exists. In Anselm's original version, it is the having of the concept of God that yields knowledge of God's existence. To understand what God is is to acknowledge that God exists. Kant later commented that this argument is not at all persuasive, but is invoked only by those who already believe that God exists.
According to Anselm, the concept of God is the concept of the being than which none greater can be conceived. To understand this concept, we need to be able to compare the greatness of concepts. One way to do so (call it greatness of scope), is to compare concepts in terms of their inclusiveness. A concept which includes more is greater. Thus my concept of a wonderful beach with an adjacent golf course is greater than my concept of just the beach.
But Anselm was working with a less straightforward comparison of concepts. We may compare two concepts which are absolutely alike, except that the object of one of the two exists, while that of the other does not. In the example from the TV script quoted last time, there are two concepts of an apple, equally firm, ripe, juicy, green, etc. But one of the two exists, while the other does not. That concept whose object exists is the greater.
If this makes sense, then we can compare the concept of a God which exists with an identical one, according to which God does not exist. The former is greater than the latter. Then the concept of a non-existence of God is not of something greater than which none can be conceived. Thus, the concept of God includes God's existence.
But Aquinas pointed out that it does not follow from this that God exists. Even if the concept of God does include God's existence, all that follows is that of concept of God is of a certain sort. We cannot think of God without thinking of God existing. But this is just a fact about how we can think. It does not mean that God must exist.
Aquinas nonetheless believed that God's existence can be demonstrated through reason. He endorsed Aristotle's argument for the unmoved mover. To avoid an infinite regress of moved movers, there must be a mover which is not itself moved. (There are two ways to understand the regress. One is that without the unmoved mover, there is no explanation of the fact that something moves now. The second is to say that without the unmoved mover, no movement would ever begin.)
More generally, Aquinas held that we can infer from features of the world to the existence of God. Such arguments are called a posteriori, because they come "after the fact" that the world exists in a certain way. Anselm's argument, on the other hand, is a priori, depending as it does on our merely possessing the conception of God.
A generalization of the unmoved mover argument is that inferring the existence of an uncaused cause. What is the cause of the existence of the world as a whole? If its cause has a cause, the same question can be asked, to infinity. So there is an uncaused cause.
A related argument is from the contingency of the world. The world might or might not have existed. Why does it exist rather than not? If it depends on something that itself might or might not exist, the same question can be asked, to infinity. So there is a necessary being, one for whom it is false that it might not have existed.
A final argument (we omit one other one) is from the order in the world. There must be, it claims, something ordering the world to account for the adaptation of things. This argument has the advantage of introducing intelligence into the ultimate being. The conclusion of the other arguments yield nothing which suggests that the being it requires is a thinking being.
Nonetheless, Aquinas did not wish to push the conclusions of reason too far. Even if we attribute intelligence to God, it bears only a remote resemblance to human intelligence. In fact, the attributes of God are only barely known to us. We must have faith to supplement reason in these matters.
Aquinas is perhaps the best known, but certainly not the only major philosopher of the medieval period. Among the many notable philosophers were John Duns Scotus and William of Ockham, who debated the nature of universals, carrying on the discussion initiated by Plato (whose works were then lost) and continued by Aristotle.