**Functional Decision Theory** is a decision theory developed by Eliezer Yudkowsky and Nate Soares which says that agents should treat one’s decision as the output of a ﬁxed mathematical function that answers the question, “Which output of this very function would yield the best outcome?”. It is a replacement of Timeless Decision Theory, and it outperforms other decision theories such as Causal Decision Theory (CDT) and Evidential Decision Theory (EDT). For example, it does better than CDT on Newcomb’s Problem, better than EDT on the smoking lesion problem, and better than both in Parﬁt’s hitchhiker problem.

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