His answer isn't random. It's based on his knowledge of apple trees in bloom (he states later that he assumed the tree was an apple tree in bloom). If you knew nothing about apple trees, or knew less than he did, or knew different but no more reliable information than he did, or were less able to correctly interpret what information you did have, then you would have learned something from him. If you had all the information he did, and believed that he was a rationalist and at the least not worse at coming to the right answer than you, and you had a different estimate than he did, then you still ought to update towards his estimate (Aumann's Agreement Theorem).
This does illustrate the point that simply stating your final probability distribution isn't really sufficient to tell everything you know. Not surprisingly, you can't compress much past the actual original evidence without suffering at least some amount of information loss. How important this loss is depends on the domain in question. It is difficult to come up with a general algorithm for useful information transfer even just between rationalists, and you cannot really do it at all with someone who doesn't know probability theory.
Today's post, "I don't know." was originally published on 21 December 2006. A summary (taken from the LW wiki):
Discuss the post here (rather than in the comments to the original post).
This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was The Modesty Argument, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
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