Today's post, was originally published on 04 April 2009. A summary (taken from the LW wiki):

The optimality theorems for probability theory and decision theory, are for perfect probability theory and decision theory. There is no theorem that incremental changes toward the ideal, starting from a flawed initial form, must yield incremental progress at each step along the way. Since perfection is unattainable, why dare to try for improvement? But my limited experience with specialized applications suggests that given enough progress, one can achieve huge improvements over baseline - it just takes a lot of progress to get there.

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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 Rationality is Systematized Winning, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.

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