Augustine says that our hearts are restless until they rest in God. This is a desire for God, but it is not at all explicit, which is why humans restlessly seek after other things, hoping to satisfy the desire, unaware that it is a desire for God. In this way, it is like hunger in a young child: hunger is a desire for food, but the child may only know that she is miserable, and not that what she desires is food. I shall call this kind of desire "deep theological desire". The argument form now is this:
- (Premise) Every desire has an intentional object.
- (Premise) If there is no God, deep theological desire has no intentional object.
- (Premise) Deep theological desire exists.
- Therefore, God exists.
Premise (1) is a consequence of the standard view of desire which entails that a desire is a state that inclines one in the direction of the intentional object. Observe, that the object is only intentional, so one can have desires for non-existent things. (If presentism were true, such desires would be very common.) I am sceptical of aspects of the standard view of desire, but I say that (1) is still correct. Premise (3) is justified by the lived experience of attentive persons like Augustine.
The really controversial assumption is premise (2), and I haven't said anything in favor of it yet. It would be mistaken to try to derive (2) from some premise like: "The intentional object of a desire has to exist", since one can desire a golden mountain. In general, it is possible to have desires with non-existent objects. What is special in the theological case?
Here is one line of thought:
- (Premise) If there can be no God, deep theological desire has no intentional object.
- (Premise) If there is no God, there can be no God.
- Therefore, if there is no God, deep theological desire has no intentional object.
So now we're down to having to argue for (5). The general schema for arguments for (5) is this:
- (Premise) All desires of type K are such that their intentional object can exist.
- (Premise) Deep theological desire has God as an intentional object.
- (Premise) Deep theological desire is a desire of type K.
- Therefore, if there can be no God, deep theological desire has no intentional object.
What we now need to do is to identify types K that make both (8) and (10) plausible. Todd Buras and Michael Cantrell have explored (8) (in a slightly different context; they were working with the desire for happiness, and a subsidiary argument that happiness is only possible if there can be a God). They even considered making K include all desires. There is some plausibility to this. After all, a desire motivates one to do actions that promote it. But an impossible object, it seems, is not promoted by anything. An obvious kind of counterexample, though, is a mathematician who desires to prove p, but unbeknownst to her p is in fact false (and hence—we hope—incapable of proof). However, Buras and Cantrell have tried to handle this kind of counterexample by saying that this is more a wish than a desire. However, I think that then the difficulty shifts to (10) (or maybe to (3), if one takes deep theological desire to be a desire by definition): why isn't deep theological desire a mere wish? I think there are resources for an answer here. A mere wish is conceptually articulated. A desire for food can be deep and unarticulated, but a mere wish seems to be more a creation of language or discursive thought. But deep theological desire is not conceptually articulated, or at least not always so—that is why it is sometimes not recognized as a desire for God. A different move that Buras and Cantrell have tried is to make K be "natural": all natural desires have possible objects. I think (8) has plausibility then, but (10) has a theological problem for Christians: the desire for God may itself be a gift of grace, and hence not "natural".
Let me try a different kind of move. Say that a desire is "visceral" provided it is in itself not formulated discursively so its intentional object is not constructed out of other ideas (here, think of how Hume thinks complex ideas are made up of simple ones). Hunger and thirst are visceral. Let K be "visceral". (Observe that it is normal for people to talk of a hunger or thirst for God, which supports the idea that the desire for God is visceral.) A visceral desire can become the subject of reflection and experimentation, and then we can find out what its object actually is. Early in life we find out that the intentional object of hunger is food and of thirst is drink (or eating food and drinking drink—I shall not worry about the distinction here, and in the case of God). Augustine's great existential discovery was that the intentional object of what I have called deep theological desire is God. So, (10) is plausible.
What about (8)? I think so. Here is a line of thought on this. What makes a visceral desire D be a desire for x? Roughly, it is that, necessarily, when an agent y who has D gets x, y's desire D is satisfied by x. But this condition is trivially satisfied if x is impossible. To make it non-trivial, we have to say:
- A visceral desire D is for x if and only if x is possible to get, and, necessarily, when an agent y who has D gets x, y's desire D is satisfied by x.
Here is the line of thought. A conceptually articulated desire gets its intentionality from the intentionality of the concepts in terms of which the desire is formulated. If I desire to prove that every even number greater than two is the sum of two primes, that desire gets its intentionality from the intentionality of my concepts of evenness, primeness, etc. Not so in the case of a visceral desire. There we need a different condition, like (12).
Another answer is that what makes hunger a desire for food is that hunger is a state with a certain teleology—a teleology directed at food. But how could there be a non-conceptually articulated teleology directed at something impossible? One account of teleology that we have is evolutionary. But there can be no evolutionary teleology directed at something impossible, since evolutionary teleology is based on the fact that the object of the teleology has in fact contributed to fitness. But that requires that the object be possible. Another account of teleology is agential. But that would require us to be designed, and our best design-based theory is theism, and hence even if we do not get an argument (12), we still get an argument for (4). Finally, there is Aristotelian teleology: things have natures, and their nature has a certain kind of fulfillment. The fulfillment is a kind of final cause of the development—they develop in order to get to the fulfillment. But it does not seem possible to have an Aristotelian teleology directed at something impossible—for in what direction would the organism be progressing if it were progressing towards something impossible? A square circle is also triangular (argument: a square circle has at least three sides, because it's a square; it has at most three sides because it's a circle; hence, it's a triangle). So, to make a square circle, should I start by making a square, a circle or a triangle? (Or by doing nothing at all, since a square circle, if it existed, would also be nothing at all, since nothing is both square and circular?)
Here is one final suggestion: Maybe a desire can get an object from society. You desire A non-viscerally, and in imitation of you, I get a visceral desire for A. However, it is not clear that my desire is actually visceral. It may, instead, be a conceptually articulated desire for that which you desire. Moreover, I think the social account does not match the phenomenology Augustine describes. Augustine isn't just imitating other people—the need is really there, deep in his heart, rather than inherited in the way we may inherit a "need" for TVs and telescopes from others.