Oppy has strangely little to say about atheistic arguments. He spends a section (3.9) considering Quentin Smith's atheological argument from cosmology. (I don't think Smith's suggestion holds up from the point of view of physics, and Oppy doesn't think it holds up philosophically.) He has a whole chapter on arguments from evil. (I don't find these interesting or convincing, so I'm going to skip them.) And he addresses some general atheological arguments in his opening chapter in a discussion of agnosticism, and (350 pages later!) includes a discussion of Clifford's Principle in his conclusion. Also, in the first chapter, he deals with the Evil God arguments like the one used by Stephen Law in his recent debate with William Lane Craig. Apart from these, he only mentions atheological arguments in passing.
(Oppy states at the beginning of the book that he's not going to consider arguments that claim the very concept of an OCMOG is incoherent. He's saving these for another book.)
So there's no discussion at all of what I think is the strongest point in favor of atheism - what I think of as the Tooth Fairy Argument. The closest he comes to it when he addresses some general principles that the atheist might try to call on in those first and last chapters. Let's take a look.
The atheist might appeal to the principle that
in the absence of any positive evidence for the existence of x's, one is rationally required to believe that there are no x's.Oppy claims that this principle is refuted by the lottery paradox. He asks us to consider a lottery in which an infinite number of tickets are sold, and only one ticket wins.
If I believe of each ticket that it won't win, then I shall be obliged to conclude that no ticket will win - that is, I will be obliged to believe something false.
According to Oppy, the existence of god(s) is like the lottery: we know that some existence proposition must be true, but since there are infinitely many such propositions (no god, god 1, god 2, etc) there is only an infinitesimal chance of any one of them being true. So the agnostic is justified in rejecting the "no god" hypothesis along with all the others.
Now, this is a very strange argument for Oppy to make, because it relies on making probability statements about an infinite set of possibilities. The problem is simply not mathematically well-defined. Elsewhere (in his chapter on fine tuning) Oppy shows himself to have a very sophisticated understanding of the difficulties of such statements. But here, without batting an eye, he allows the agnostic to cite the lottery paradox as if it were unproblematic.
He concludes his one-paragraph discussion like this:
Thus, for example, although there is no good reason to think that there are currently intelligent beings inhabiting the fifth planet of the Vega system, the correct view to have is simply that this claim is a very unlikely one.That is, (complete) lack of evidence is not enough to make us disbelieve in something, it is only enough to make that something very unlikely.
I think this is correct in a strictly logical sense, as far as it goes. But I also think it doesn't go far enough. It's not just that we are lacking in evidence for a god, it's that we have positive reasons to believe that something like a god can't possibly exist. Rather than "intelligent life on Vega 5," the analogy should be "purple elephants on the planet Pluto." We know elephants can't exist on Pluto, because mammals need food, oxygen, and liquid water, and none of that is available there.
Likewise for gods. It's not just that we're lacking evidence for them: we have good reason to think that intelligence requires a complex neural system (or something like it - as in a computer). The evidence is both experimental and theoretical. Experimentally, all known intelligent beings have complex neural systems. And theoretically, the ability to process information requires some such system.
But according to theists, God doesn't have any neurons. He doesn't have a material body at all. In fact, the OCMOG has no moving parts. How then, can God be intelligent?
Gods are as unlikely as purple Plutonian pachyderms.