Instead, the “assigned” prior (the one that the assumption refers to) must actually be an agent’s pre-prior updated by nature’s assignment of priors. Only then would updating the pre-prior by this “assigned” prior give you back the “assigned” prior. (I put “assigned” in quotes, because these priors are not actually assigned in the normal sense of the word.)

At this point it becomes clear that Robin’s other assumption, that each pre-agent does not think his “assigned” prior has a special origin, is equivalent to saying that all pre-agents have the same pre-prior. After all, if you have a pre-prior different from everyone else, then your “assigned” prior is special, since it is just the updated version of your special pre-prior.

Human beings are not born as generic reasoners without any information about the world we live in. Instead evolution has provided us with a prior that is partially optimized for this world. However since evolution is random and unfinished, some aspects of this prior are arbitrary. Robin’s idea is that we can remove the randomness and keep only the useful information in the prior by taking the nature-provided prior as the first data point of a generic reasoner with a generic prior (which Robin calls a pre-prior, and which truly has no information about the world), instead of adopting it directly as the prior.

If I’ve understood it correctly so far, my remaining question is, is it assumed that all of the pre-agents have the same pre-prior? It seems to me that this updating process will remove the arbitrary differences in the nature-provided priors, but differences in the pre-priors continue to be reflected in the post-update priors. Is that correct?

Now, you might think that the reasons for denying “Robin(t) is identical to Robin(t+s)” are bad ones, but I’d suggest you read Parfit on this. In any event, you’re a far cry from establishing the contradiction you claim.