Even the person who denies that God exists claims to know what it means to say that God does not exist. That is, in denying that God exists, even the atheist acknowledges that by "God" is meant the supreme being; it's only that, as far as the atheist is concerned, such a supreme being does not exist. Instead of calling God the supreme being, St. Anselm of Canterbury (1033-1109) says that even an atheist, therefore, would agree that by "God" people mean that being than which nothing greater can be conceived.
Anselm then adds a second observation: wouldn't a being that exists both in reality and as a concept in people's minds be greater than that existed only in people's minds? For example, wouldn't a winged horse that really existed be a greater being than one that existed only in people's minds? That is, it seems that something that exists both outside the mind (extra-mentally) plus in the mind is greater than something that exists only in the mind. An idea of having $100 is certainly not as great as both having the idea and actually having the $100. Furthermore, a being that must exist (that is, one that exists necessarily ) is greater than one that exists contingently (that is, depending on another for its existence).
So if God is that being than which nothing greater can be conceived, then God cannot be some being who exists only mentally (as someone's idea), because there would be a being who is greater--namely, one who exists extra-mentally and mentally. In addition, a being who exists without relying on any other being for its existence would be better than one that exists only insofar as it relies on another. So the only appropriate way to think of God is as a being who exists necessarily, both mentally and extra-mentally. And if God's existence (by definition) means that he exists not only in people's minds but also outside people's minds, then God must exist outside people's minds: that is, God exists necessarily. In summary:
1. Everyone acknowledges that by "God" we mean that being than which nothing greater can be conceived.
2. But a being that exists only in the mind is not as great as a being that exists in the mind as well as in reality. Real existence + conceptual existence > conceptual existence alone.
3. Therefore, God must exist both in the mind and in reality, since a God who existed only in the mind would not be that being than which nothing greater could be conceived. Since God is always on the left of the greater-than sign, God must really exist.
Notice this about the argument: it does not start from our experience of the universe; it starts simply from the meaning of the notion of God . If you know what the concept God means, you must conclude that God exists. If you don't, you don't know what you are talking about in talking about God.
Objections :
1. Gaunilo and Thomas Aquinas: imagining that there is a perfect something (say, an island) doesn't mean that such a thing exists. It only means that one can imagine such a thing.
2. David Hume: If you can conceive of something (e.g., God's non-existence), then it is not a logical contradiction. If you can think of God as existing, you can also think of God as not existing. But if it is possible to think that God might not exist, then that is all that is needed to show that the ontological argument fails, because it is always illegitimate to think that from a definition, idea, or statement of meanings you can make claims about reality (which must always be based on observation, not reason alone).
3. Hume again: from the meaning of the idea of God we cannot conclude anything about whether the thing that the idea is an idea of exists. We could have a meaningful idea of a solid gold mountain, but that would not entail that such a mountain exists.
4. Kant: by saying that God exists or God does not exist, we do not add anything or subtract anything from our understanding of God; it only affirms or denies that there is something that I think when we use the word "God." In short, existence is not a predicate or attribute of anything. To imagine or think of anything presupposes that the thing exists as the object of thought; but to imagine or think that the thing exists in no way changes what one thinks: it still is the object of what one is thinking. Instead of saying that existence in reality + existence in the mind is greater than existence of the mind alone, Anselm should have said that the concept of existence in reality + the concept of mental existence is greater than the concept of mental existence. But if he said that, he would recognize that his argument never moves beyond concepts, which means that God's real existence is never actually proved.
Anselm's reply: having an idea of God is unlike any other idea we can have. Any idea one has about a perfect island or a solid gold mountain can be superseded by a more perfect idea. But no one can think of something more perfect than God, because that more perfect thing would itself be (by definition) God. So if you think of something that might not exist, you are not really thinking about God. In this sense, the idea of God is unlike any other idea we could have.
Besides, Anselm's purpose in all of this is not to prove to a non-believer that God exists. Rather, it is intended to show the believer how his or her belief in God makes sense.
II. The Moral Argument (developed by Kant): No one can theoretically prove that God exists either a posteriori or a priori . Even if such arguments are clever, they will never be enough to persuade someone to believe in God. What is needed for that is not a theoretical argument as much as a practical argument, a moral reason to believe in God. This argument shows that without God, there is no good reason to make moral distinctions or to strive to be moral. Here is how that argument goes:
1. If we assume that there is a real difference between good and evil and that some things are better than others, we have to assume as well that there is some standard on which such judgments are based. We can refer to the thing on that standard that is "the best" as the summum bonum (Latin for "the greatest good," the best thing that anyone could do).
2. Whatever the greatest good would be, it certainly would include both virtue and happiness: that is, the greatest good would be not only a situation in which people do what they ought to do (i.e., act virtuously) but also a situation in which they are happy. Acting virtuously is something we have control over, but we do not have control over whether we are happy (since sometimes bad things happen to us over which we have no control).
3. If morality makes sense, then, it is only because there is a greatest good; and there is a greatest good only if virtue and happiness are united in it. There must be some agency that links the worthiness of being happy (virtue) with actual happiness--either in this life or in the next--and that agent is God. So, if morality makes sense, then God must exist.
Objections :
Maybe morality does not make sense, in which case there is no reason to believe in God.
We do not need to believe in God to provide the motivation to do good; concern for one another is (or should be) enough to motivate people to act morally.
III. Moral Implications of Believing in God
(a) Traditional religious morality : Belief in God requires that one follow certain moral rules.
Objections : such rules are often out of date, ignore motives or particular circumstances, and are unconvincing to people who learn from experiences rather than simply accepting rules because they are told to do something.
(b) Social gospel/liberation theology : the primary goal of religion is to improve the social and political conditions of others.
Objection : human sinfulness can never completely overcome oppression and injustice.
(c) Contextual (Situation) ethics : ethical rules cannot substitute for a loving Christian concern for persons in real situations.
Objection : intuitive, free responses are often excuses for personal preferences, not morality.