I get why in examples where you can see each others' source code this can be the case, and I do one-box on Newcomb where a similar situation is given, but I don't see how we can presume that there is this kind of instrumental value. All we know about this person is he is a flat earther, and I don't see how this corresponds to such efficient lie detection in both directions for both of us.

What does the source code really impart? Certainty in the other process' workings. But why would you need certainty? Is being a co-operator really so extraordinary a claim that to support it you need overwhelming evidence that leaves no other possibilities?

The problem is that there are three salient possibilities for what the other player is:

• Defector, who really will defect, and will give you evidence of being a defector
• Co-operator, who will really cooperate (with another who he believes to be a co-operator), and will give you evidence of being a co-operator
• Deceiver, who will really defect, but will contrive evidence that he is a co-operator

Between co-operator and deceiver, all else equal, you should expect the evidence given by co-operator to be stronger than evidence given by deceiver. Deceiver has to support a complex edifice of his lies, separate from reality, while co-operator can rely on the whole of reality for support of his claims. As a result, each argument a co-operator makes should on average bring you closer to believing that he really is a co-operator, as opposed to being a deceiver. This process may be too slow to shift your expectation from the prior of very strongly disbelieving in existence of co-operators to posterior of believing that this one is really a co-operator, and this may be a problem. But this problem is only as dire as the rarity of co-operators and the deceptive eloquence of deceivers.