This relates to my recent post on existence in many-worlds.
I care about possible people. My child, if I ever have one, is one of them, and it seems monstrous not to care about one's children. There are many distinct ways of being a possible person. 1)You can be causally connected to some actual people in the actual world in some histories of that world. 2)You can be a counterpart of an actual person on a distinct world without causal connections 3)You can be distinct from all actual individuals, and in a causally separate possible world. 4)You can be acausally connectable to actual people, but in distinct possible worlds.
Those 4 ways are not separate partitions without overlap, sometimes they overlap, and I don't believe they exhaust the scope of possible people. The most natural question to ask is "should we care equally about about all kinds of possible people". Some people are seriously studying this, and let us hope they give us accurate ways to navigate our complex universe. While we wait, some worries seem relevant:
1) The Multiverse is Sadistic Argument:
P1.1: If all possible people do their morally relevant thing (call it exist, if you will) and
P1.2: We cannot affect (causally or acausally) what is or not possible
C1.0: Then we cannot affect the morally relevant thing.
2) The Multiverse is Paralyzing (related)
P2.1: We have reason to care about X-Risk
P2.2: Worlds where X-Risk obtains are possible
P2.3: We have nearly as much reason to worry about possible non-actual1 worlds where X-risk obtains, as we have to actual worlds where it obtains.
P2.4: There are infinitely more worlds where X-risk obtains that are possible than there are actual1
C2.0: Infinitarian Paralysis
1Actual here means belonging to the same quantum branching history as you. If you think you have many quantum successors, all of them are actual, same for predecessors, and people who inhabit your Hubble volume.
3) Reality-Fluid Can't Be All That Is Left Argument
P3.1) If all possible people do their morally relevant thing
P3.2) The way in which we can affect what is possible is by giving some subsets of it more units of reality-fluid, or quantum measure
P3.3) In fact reality-fluid is a ratio, such as a percentage of successor worlds of kind A or kind B for a particular world W
P3.4) A possible World3 with 5% reality-fluid in relation to World1 is causally indistinguishable from itself with 5 times more reality-fluid 25% in relation to World2.
P3.5) The morally relevant thing, though by constitution qualitative, seems to be quantifiable, and what matters is it's absolute quantity, not any kind of ratio.
C3.1: From 3.2 and 3.3 -> We can actually affect only a quantity that is relative to our world, not an absolute quantity.
C3.2: From C3.1 and P 3.5 -> We can't affect the relevant thing.
C3.3: We ended up having to talk about reality fluid because decisions matter, and reality fluid is the thing that decision changes (from P3.4 we know it isn't causal structure). But if all that decision changes is some ratio between worlds, and what matters by P3.5 is not a ratio between worlds, we have absolutely no clue of what we are talking about when we talk about "the thing that matters" "what we should care about" and "reality fluid".
These arguments are here not as a perfectly logical and acceptable argument structure, but to at least induce nausea about talking about Reality-Fluid, Measure, Morally relevant things in many-worlds, Morally relevant people causally disconnected to us. Those are not things you can Taboo the word away and keep the substance around. The problem does not lie in the word 'Existence', or in the sentence 'X is morally relevant'. It seems to me that the service that that existence or reality used to play doesn't make sense anymore (if all possible worlds exist or if Mathematical Universe Hypothesis is correct). We attempted to keep it around as a criterial determinant for What Matters. Yet now all that is left is this weird ratio that just can't be what matters. Without a criterial determinant for mattering, we are left in a position that makes me think we should head back towards a causal approach to morality. But this is an opinion, not a conclusion.
Edit: This post is an argument against the conjunctive truth of two things, Many Worlds, and the way in which we think of What Matters. It seems that the most natural interpretation of it is that Many Worlds is true, and thus my argument is against our notion of What Matters. In fact my position lies more in the opposite side - our notion of What Matters is (strongly related to) What Matters, so Many Worlds are less likely.