Indeed, for hypotheses like “the sun will rise tomorrow,” it’s not clear what it means for that hypothesis to be “true,” since it depends on some model of “the sun”, “will rise”, and “tomorrow.” I think this is a common domain for philosophical debate, but I know little about that debate.

I don’t think you should view sentences in isolation as things that get truth values. My take is that you have a few better-defined alternatives: you could talk only about your own direct experiences, you could talk about mathematical facts, or you could talk about statements about the world which don’t make any ontological assumptions (where the last one is still philosophically fraught).

So you might have a belief like “I won’t see the sun fail to rise tomorrow,” which can be defined purely in terms of your own experiences (in a way that is robust to ontology shifts like the one you described—a formal definition of “see the sun rise tomorrow” could be applied directly to possible worlds where you discover you are in the matrix, to pass judgment on whether or not you saw the sun rise, after you woke up from the matrix). Or you might have a belief like “in simple computer programs, observers like me live in bits of code that can be functionally modeled like ,” which is a mathematical fact (where involves a model of the earth and sun where the sun doesn’t rise tomorrow). Or you could have a belief of the form “The universe is made of space full of fields, and some of them are related to each other like .”

In any of those cases, learning you are in the matrix would just cause your belief to be automatically false.

If so, I am confused about how one goes about even determining the meaning of H, let alone judging whether or not it is true, in sufficiently strange possible worlds, and maybe you can clear up this confusion for me. Suppose H = “the sun will not rise tomorrow” and that I assign to this hypothesis an extremely low probability. Tomorrow I observe that the sun has not risen. This appears to be evidence that the sun has not risen, but you say to me “aha, it’s actually evidence that you are living in the Matrix and the Matrix Lords are screwing with you, the actual sun totally rose.”

Okay, but if I’ve lived in the Matrix my entire life, then I learned the word “sun” in the Matrix, and if I dereference that pointer it actually refers to the part of the Matrix that simulates the actual sun, and that part really didn’t do the operation that corresponds to rising! I have never seen the actual sun. There is nothing in my current vocabulary that can reasonably be said to refer to the actual sun. So in what sense is H false here?