Theories of teleology have a few problems. First, theories of teleology often wind up drawing causal arrows from the future to the past. It also leads you to make predictions based on anthropomorphism. Finally, it opens you up to the Mind Projection Fallacy, assuming that the purpose of something is an inherent property of that thing, as opposed to a property of the agent or process that produced it.
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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 Magical Categories, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.