Let's start with an example: my length-in-meters, along the major axis, rounded to the nearest integer, is 2. In this statement "2", "rounded to nearest integer", and "major axis" are clearly mathematical; while "length-in-meters" and "my (me)" are not obviously mathematical. The question is how to cash out these terms or properties into mathematics.
We could try to find a mathematical feature that defines "length-in-meters", but how is that supposed to work? We could talk about the distance light travels in 1 / 299,792,458 seconds, but now we've introduced both "seconds" and "light". The problem (if you consider non-mathematical language a problem) just seems to be getting worse.
Additionally, if every apparently non-mathematical concept is just disguised mathematics, then for any given real world object, there is a mathematical structure that maps to that object and no other object. That seems implausible. Possibly analogous, in some way I can't put my finger on: the Ugly Duckling theorem.
There are at least ten different conceptions of how the World can be made of many worlds.
But are those just definitional disputes? Or are they separate claims that can be evaluated. If they are distinct, in virtue of what are they distinct. Finally, do we have good grounds to care (morally) about those fine distinctions?
Max Tegmark's taxonomy is well known here.
Brian Greene's is less, and has 9, instead of four, kinds of multiverse, I'll risk conflating the Tegmark ones that are superclasses of these, feel free to correct me:
I don't understand branes well enough (or at all) to classify the others. The holographic one seems compatible with a multitude, if not all, previous ones.
Besides all those there is David Lewis's Possible Worlds in which all possible worlds exist (in whichever sense the word exist can be significantly applied, if any). For Lewis, when we call our World the Actual World, we think we mean the only one that is there, but what we mean is "the one to which we happen to belong". Notice it is distinct from the Mathematical/Ultimate in that there may be properties of non-mathematical kind.
So Actuallewis= Our world and Actualmost everyone else=Those that obtain, exist, or are real.
The trouble with existence, or reality, is that it is hard to pin down what it is pointing at. Eliezer writes:
and elsewhere
Now another interesting way of looking at existence or reality is
Reality=I should care about what takes place there
It is interesting because it is what is residually left after you abandon the all too stringent standard of "causally connected to me", which would leave few or none of the above, and cut the party short.
So Existenceyud and Existencemoral-concern are very different. Reality-fluid, or Measure, in quantum universes is also different, and sometimes described by some as the quantity of existence. Notice though that the Measure is always a ratio - say these universes here are 30% of the successors of that universe, the other 70% are those other ones - not an absolute quantity.
Which of the 10 kinds of multiverses, besides our own, have Existenceyud Existencemoral-concern and which can be split up in reality-fluid ratios?
That is left as an exercise, since I am very confused by the whole thing...