  This paper describes and motivates a new decision theory known as functional
decision theory (FDT), as distinct from causal decision theory and evidential
decision theory. Functional decision theorists hold that the normative
principle for action is to 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?" Adhering to this principle delivers a
number of benefits, including the ability to maximize wealth in an array of
traditional decision-theoretic and game-theoretic problems where CDT and EDT
perform poorly. Using one simple and coherent decision rule, functional
decision theorists (for example) achieve more utility than CDT on Newcomb's
problem, more utility than EDT on the smoking lesion problem, and more utility
than both in Parfit's hitchhiker problem. In this paper, we define FDT, explore
its prescriptions in a number of different decision problems, compare it to CDT
and EDT, and give philosophical justifications for FDT as a normative theory of
decision-making.
