In this blog post I shall be addressing the question "What exists?". This is a thorny issue and before going further I must warn the reader that my thoughts on this matter are in a state of flux (hence I may soon change my mind on these matters). Never the less I shall present the state of my thinking on this.
1) There is some way of representing universes as mathematical entities (specifically infinite sequences in finitary languages (for mathematicians I am assuming AC)). My basis for this is simple: I regard mathematics as the study of all possible universes.
If mathematics does not allow the representing of universes (or at least their modelling for the questions in hand) then I regard mathematics to be incorrect or deficient in some way.
2) Two universes should be regarded as the same if their representations can be calculated from each other (in an objectively computational way). If universe A is represented 01010201... and universe B is represented 0011001100220011... then there is a simple way to calculate one from the other & vice versa and so universes A and B should be considered identical. This is a decidedly strong principle and should not be accepted without some deep thought. It has some interesting consequences in ethics and aesthetics (perhaps another post topic).
This would imply that all finite universes are essentially the same (a very strong claim!). I don't have any issue with this personally but you would if you believed that infinite universes were impossible (and perhaps for other reasons too). It does not imply that all infinite universes are the same though.
3) There are two notions of existence. Firstly what entities exist somewhere out there. Secondly what entities exist that could in principle effect you.
3a) Consider all possible universes. The question should be: "Which of these possible universes exist?". Now if some possible universes exist and some do not we can ask "Why do some exist and others not?". Either there is some logical difference between the contents of these possible universe or there is not. If there is no logical difference then the fact of the existence of those universes is entirely arbitrary (A bad position I'd hope you agree). If there is a logical difference we are asked to accept that the presence of some feature of possible universes makes them exist (say the property of having cats alive at some point in their history). Again any such internal feature conferring existence if highly dubious. It seems evident to me that under these conditions that in fact all possible universes should be taken to exist or all possible universes must be taken not to exist. From these two possibilities I'd plump for the former!
3b) The second notion of existence can be defined roughly as follows: Everything that has effected you or might at some future time effect you must be taken to exist. Everything else does not exist.
Now for the punch line: The second notion of existence is generally the most useful. After all the tiger thats walking down the street outside your front door in a parallel universe is of no immediate concern to you. However, when engaged in discussions concerning the probability of our universe the first definition is key. Hence my answer to the question of why our universe has the apparent fine tuning it has is "All possible universes exist so there is nothing to explain."
4) Seeing a universe from a perspective is like projecting that universe down in some way. Imagine the universe as a wire model and your perspective as the shadow it casts on the table top. Different ways of representing a universe are different universal perspectives.
A word of caution: This does not imply moral relativism. The moral decisions you make must still be made from your perspective (although not necessarily with just any moral theory).
I may put up another post soon arguing why I think this attitude to the concept of existence implies that qualia don't exist and that the strong AI thesis is probably correct.