This post is a follow up to a previous one on rationality and deceit. I will presuppose familiarity with that discussion, including the comments.
I obviously accept Duck's point that Mr. Z is rational in the more robust sense that he has appropriately used his experiences to draw a certain conclusion relative to background theory. The "irrationality" lies in the background theory somewhere. (Note that my intuitions change if we assume Mr. Z is in an X-files type circumstance where his belief in the connection itself is rational.)
In CO's example, I have a clear intuition that the Nazis were being deceived. So whatever difference there may be between the "ersatz evidence" in the original case and the evidence in the Nazi case, it can't lie in the fact that the experiences are staged. PERHAPS the answer lies in this vicinity: in order for m to be evidence that p, m must mean (either naturally or non-naturally) p. [I don't think this is quite right, see below.] But by implicit assumption, black cats don't naturally mean alien abductions. And clearly the circumstances are such that I don't meanNN by releasing the cats that alien abductions are occurring in Roswell. In contrast, the feints by the Allied forces do naturally mean (or at least it is reasonable to so take them as naturally meaning) that the real invasion will take place at Callais.
Now we clearly have a distinction between what some event naturally means and what it is reasonable to take that event to naturally mean. Its being reasonable to take m to mean p is neither necessary nor sufficient for m's meaning p. So perhaps the difference between genuine evidence and ersatz evidence is this:
m is genuine evidence that p for x iff it is reasonable for x to take m to mean (naturally or non-naturally) p; otherwise, m is ersatz evidence.
(Note: this definition predicts that it is possible for m to be ersatz evidence that p for x even if m really does mean p! Is that right? I think so. Retell the original example in such a way that black cats DO mean alien abductions, but Mr. Z is in no way reasonable in thinking that they do; the correctness of his believe is just blind luck.)
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