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Wei_Dai comments on Decision Theories: A Less Wrong Primer - Less Wrong
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It's pretty easy to explain the main innovation in TDT/UDT/ADT: they all differ from EDT/CDT in how they answer "What is it that you're deciding when you make a decision?" and "What are the consequences of a decision?", and in roughly the same way. They answer the former by "You're deciding the logical fact that the program-that-is-you makes a certain output." and the latter by "The consequences are the logical consequences of that logical fact." UDT differs from ADT in that UDT uses an unspecified "math intuition module" to form probability distribution over possible logical consequences, whereas ADT uses logical deduction and only considers consequences that it can prove. (TDT also makes use of Pearl's theory of causality which I admittedly do not understand.)
There is no need to focus on these concepts. That the fact of decision is "logical" doesn't usefully characterize it: if we talk about the "physical" fact of making a decision, then everything else remains the same, you'd just need to see what this physical event implies about decisions made by your near-copies elsewhere (among the normal consequences). Likewise, pointing to a physical event doesn't require conceptualizing a "program" or even an "agent" that computes the state of this event, you could just specify coordinates in spacetime and work on figuring out what's there (roughly speaking).
(It's of course convenient to work with abstractly defined structures, in particular decisions generated by programs (rather than abstractly defined in a more general way), and at least with mathematical structuralism in mind working with abstract structures looks like the right way of describing things in general.)
But how does one identify/encode a physical fact? With a logical fact you can say "Program with source code X outputs Y" and then deduce consequences from that. I don't see what the equivalent is with a "physical" notion of decision. Is the agent supposed to have hard-coded knowledge of the laws of physics and its spacetime coordinates (which would take the place of knowledge of its own source code) and then represent a decision as "the object at coordinate X in the universe with laws Y and initial conditions Z does A"? That seems like a much less elegant and practical solution to me. And you're still using it as a logical fact, i.e., deducing logical consequences from it, right?
I feel like you must be making a point that I'm not getting...
The same way you find a way home. How does that work? Presumably only if we assume the context of a particular collection of physical worlds (perhaps with selected preferred approximate location). Given that we're considering only some worlds, additional information that an agent has allows it to find a location within these worlds, without knowing the definition of those worlds.
This I think is an important point, and it comes up frequently for various reasons: to usefully act or reason, an agent doesn't have to "personally" understand what's going on, there may be "external" assumptions that enable an agent to act within them without having access to them.