Why Prefer Any Decision Theory?

People who construct scenarios where it doesn't, or take umbrage at the idea of "fair problems" are falling foul of symmetry arguments, and would never apply this level of scrutiny to any other system of making decisions.Intro Bentham's Bulldog recently posted an attempted takedown of Functional Decision Theory on Less Wrong. This is probably the second bravest post I've seen someone in the EA/Rat sphere post on Less Wrong.His first argument was that FDT is not mathematically well-defined, because logical counterfactuals are not well-understood and, as he argues, can never be well-defined. I don't know enough about the state of the logical counterfactual research, so I'll leave that to a pro decision theorist to explain.His second argument was that FDT gives the wrong answer sometimes. I think that he skips up and down different levels of demand for rigor, when talking about different decision theories. I think FDT beats CDT, or at least ties, at basically every point on the spectrum between totally abstract and totally practical.Decision Problems as a Tower of AssumptionsThe standard jumping off point for decision theories is the set of fair problems with perfect information about the overall scenario:You get to make decisionsVarious decision theorist trickster gods such as Omega get to perfectly simulate youYou know, ahead of time, everything about the situation including what kinds of simulations you might be put inThis doesn't mean you have perfect information about which of two identical situations you're in, once the situation starts, but it does mean you can have a well-calibrated Bayesian prior over it.The agent can only simulate your actions, and doesn't have access to information about your decision theoryThis includes the famous Newcomb's Problem[1]. It's worth working through Newcomb's problem in an FDT language, since that will be instructive for cases in the future. FDT reasons through