I find many worlds somewhat convincing, but I don't understand your take on it:

"But modal realism holds that the situation for induction is exactly like it is for the cubs in the hypothetical scenario. There are people exactly like me up until this point—and for an infinite number of them, induction fails one second later. "

For a start its not clear whether there are an infinite number of possible worlds or not. If we assume there are vastly many but not infinite then that statement is clearly incorrect. Additionally for that case, the overwhelming number of people/worlds induction doesn't fail one second later. The really bizarre worlds are incredibly unlikely compared to the ones where things stay normal.

Even with infinite worlds you can make an analogy to physical size. Lets say physical space can be infinitely small (I believe not but lets imagine) then you can still compare infinities, because we are quite happy to say a stick is 10cm long and its 2*5cm even though both would contain infinitely many points.

Imagine your 10cm stick represents the worlds at this moment, with the sensible ones being green, the crazy ones red. Its currently 100% green. Now go forward 1s:

The stick still appears 100% green, 10.0cm green even though it would be 9.99..... cm green and <<1e-100cm red.

Induction would still work for practically all of it.

Additionally if there is somehow a low probability cutoff, (this isn't thought to be the case with QM but it is not impossible either) then those bizarre worlds don't get actualized at all.

We don't know if it's 99.9999999999999% because modal realism only claims that all these worlds exist while remaining silent on the proportions of worlds.

It would seem you can't presume there's only one single world for each possible scenario, because that will make the proportions of worlds solely depend on our descriptions.

You have to make some assumption along those lines for probability to make sense. If you have a theory of MR that doesn't account for probability, it may well fail to account for induction as well, since the two are closely related -- but failing to account for probability is not a feature of all and any MR

"Suppose you currently have some belief, say, that the Cubs are going to win the world series. But you find out that the universe is filled with alternative earths, exactly like ours up until this point, and throughout those earths, of all the people who are exactly like you up until this point and have the same evidence of you, who bet that the Cubs will win, 99.9999999999999% of them are wrong. That would seem to undermine your justification for believing that the Cubs will, in fact, win."

1. Beliefs about whether the Cubs win refer to the actual world. If they fail to win in some possible world, that doesn't negate their winning in our world.

2. The proportions of worlds where the cubs win aren't arbitrary: they reflect the probability. If there is a high proportion of worlds where they Cubs didn't win, then it was unlikely even though it happened in our world. That doesn't change anything, because we already accept that unlikely things can happen.If it is likely that the Cubs win, there will not be a high proportion of worlds where they lose.

I don't see how modal realism is even a theory of modality. If the other worlds are just as real and concrete as ours, then they aren't "possible worlds" at all; they're just bits of the actual world which we can't get to. Surely being inaccessible to us doesn't make something count as a different possible world.

I also don't think it provides a very good explanation of why there's something rather than nothing. If somebody wants to know why this world exists, telling them that there are lots of other (equally real, concrete) worlds doesn't seem to explain a thing; in fact, it seems to make the problem worse.

It's explained in the first para of the wikipedia article:-

"Modal realism is the view propounded by philosopher David Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours."[1] It is based on four tenets: possible worlds exist, possible worlds are not different in kind from the actual world, possible worlds are irreducible entities, and the term actual in actual world is indexical, i.e. any subject can declare their world to be the actual one, much as they label the place they are "here" and the time they are "now"."

I'm aware of what modal realists think. My point is that I don't think they're actually offering a good theory of modality. It seems to me that if the other worlds are concrete, then they aren't possible, they're actual. Lewisians try to get around this by using "actual" in a novel way (i.e. as an indexical), but this is very implausible. The fact that some part of concrete reality is inaccessible to us doesn't make it another "possible world."

Everything actual is also possible, because impossible things can't be actual. The term for possible but non-actual is "potential". There's no theorem that the possible is always non-actual.

Modal Realists literally believe that Goku concretely exists, he's just not spatially connected us.

Modal Realists can't exist.

maybe conscious experience is only possible in worlds where induction works

I find many worlds somewhat convincing, but I don't understand your take on it:

"But modal realism holds that the situation for induction is exactly like it is for the cubs in the hypothetical scenario. There are people exactly like me up until this point—and for an infinite number of them, induction fails one second later. "

For a start its not clear whether there are an infinite number of possible worlds or not. If we assume there are vastly many but not infinite then that statement is clearly incorrect. Additionally for that case, the overwhelming number of people/worlds induction doesn't fail one second later. The really bizarre worlds are incredibly unlikely compared to the ones where things stay normal.

Even with infinite worlds you can make an analogy to physical size. Lets say physical space can be infinitely small (I believe not but lets imagine) then you can still compare infinities, because we are quite happy to say a stick is 10cm long and its 2*5cm even though both would contain infinitely many points.

Imagine your 10cm stick represents the worlds at this moment, with the sensible ones being green, the crazy ones red. Its currently 100% green. Now go forward 1s:

The stick still appears 100% green, 10.0cm green even though it would be 9.99..... cm green and <<1e-100cm red.

Induction would still work for practically all of it.

Additionally if there is somehow a low probability cutoff, (this isn't thought to be the case with QM but it is not impossible either) then those bizarre worlds don't get actualized at all.

We don't know if it's 99.9999999999999% because modal realism only claims that all these worlds exist while remaining silent on the proportions of worlds.

It would seem you can't presume there's only one single world for each possible scenario, because that will make the proportions of worlds solely depend on our descriptions.

edited May 15, 2023You have to make some assumption along those lines for probability to make sense. If you have a theory of MR that doesn't account for probability, it may well fail to account for induction as well, since the two are closely related -- but failing to account for probability is not a feature of all and any MR

My current take is that, MR explains probability. The real probability is about the proportion of worlds, which we lack epistemic access.

Expected probability, on the other hand, may be defined pragmatically? like what they have done in decision theory.

"Suppose you currently have some belief, say, that the Cubs are going to win the world series. But you find out that the universe is filled with alternative earths, exactly like ours up until this point, and throughout those earths, of all the people who are exactly like you up until this point and have the same evidence of you, who bet that the Cubs will win, 99.9999999999999% of them are wrong. That would seem to undermine your justification for believing that the Cubs will, in fact, win."

1. Beliefs about whether the Cubs win refer to the actual world. If they fail to win in some possible world, that doesn't negate their winning in our world.

2. The proportions of worlds where the cubs win aren't arbitrary: they reflect the probability. If there is a high proportion of worlds where they Cubs didn't win, then it was unlikely even though it happened in our world. That doesn't change anything, because we already accept that unlikely things can happen.If it is likely that the Cubs win, there will not be a high proportion of worlds where they lose.

edited May 13, 2023I don't see how modal realism is even a theory of modality. If the other worlds are just as real and concrete as ours, then they aren't "possible worlds" at all; they're just bits of the actual world which we can't get to. Surely being inaccessible to us doesn't make something count as a different possible world.

I also don't think it provides a very good explanation of why there's something rather than nothing. If somebody wants to know why this world exists, telling them that there are lots of other (equally real, concrete) worlds doesn't seem to explain a thing; in fact, it seems to make the problem worse.

edited May 13, 2023It's explained in the first para of the wikipedia article:-

"Modal realism is the view propounded by philosopher David Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours."[1] It is based on four tenets: possible worlds exist, possible worlds are not different in kind from the actual world, possible worlds are irreducible entities, and the term actual in actual world is indexical, i.e. any subject can declare their world to be the actual one, much as they label the place they are "here" and the time they are "now"."

edited May 15, 2023I'm aware of what modal realists think. My point is that I don't think they're actually offering a good theory of modality. It seems to me that if the other worlds are concrete, then they aren't possible, they're actual. Lewisians try to get around this by using "actual" in a novel way (i.e. as an indexical), but this is very implausible. The fact that some part of concrete reality is inaccessible to us doesn't make it another "possible world."

Everything actual is also possible, because impossible things can't be actual. The term for possible but non-actual is "potential". There's no theorem that the possible is always non-actual.