Despite my individualism, I actually think that fully-blown (time-slice) atomism is the place to start. However, from this perspective in the here-and-now, I realise that I care about more than just my present stage. The present stage, on its own, lacks meaning. It is only by situating my present stage within the narrative arc of a whole life that it becomes comprehensible. Likewise for each other momentary stage. So I am willing to make momentary sacrifices in order to construct a whole life that has a certain coherence and value that outstrips the mere sum of its parts. Or so I conceive of life: it's the whole thing, rather than each moment, that I find myself concerned about on reflection.
But if we go this far, why stop there? Given Parfit-style reductionism about personal identity, I don't think there's any really deep metaphysical unity between my temporal parts. (At each momentary stage, I choose to "identify" with the larger, temporally-extended whole, but this a "rationally constructed" sort of unity, rather than anything that comes built into the world itself.) On a metaphysical level, I think there is a strong formal analogy between 'my present stage' vs. 'myself' (temporally extended) and 'myself' vs. 'humanity' (inter-personally extended). Should this lead us to identify more with the latter, as a collective that in some sense transcends us all?
My initial argument is easily reapplied:
Starting from this individual perspective of mine, I realise that I care about more than just my own life. This life, on its own, lacks meaning. It is only by situating my life within the narrative arc of a whole society (/civilization) that it becomes comprehensible. Likewise for each other individual. So we are willing to make personal sacrifices in order to construct a whole society that has a certain coherence and value that outstrips the mere sum of its parts. Or so we conceive of civilization: it's the whole thing, rather than each individual, that we find ourselves concerned about on reflection.
This clears plenty of room for what I call "super-human" values -- i.e. accomplishments of humanity, such as constructing the Great Pyramids, or exploring deep space, regardless of their impact on individual welfare. It could also legitimize the (otherwise irrational) practice of caring about a socially salient mass event more than the analogous aggregate of widely distributed events. (For example, compare 9/11 to car crashes. The threat of terrorism seems to have far greater social significance for Americans, out of all proportion to the mere aggregate-individual impact.)
But there are some important disanalogies, of course. Most importantly, perhaps, there is a greater rational unity among the stages of a life than among the individuals of a society. My momentary stages are bound together by psychological continuity -- a sameness of character, values, goals, etc. There is far more diversity and conflict among the individuals in our society. Arguably, we just don't have enough in common to construct a rationally unified entity from society, as we can for an individual life. What typically happens, of course, is that the "mainstream" or majority group claim to constitute the whole society, and so freely trample over dissenting minorities. But tyranny is not community, and majority will is not the same thing as the general will.
Still, perhaps we may be led to a form of communitarianism on a smaller scale, whereby a broad political liberalism enables people to enter (and exit) niche "communities" of choice. After all, some of the deepest satisfaction we can find in life is from contributing to projects that are larger than ourselves, and that we consider to have enduring worth. These may go beyond "weak" communities of convenience and mutually-beneficial cooperation, to the kind of so-called "strong" community that is valued by its members over and above their individual interests in it. In such a case, the methodology of rational expansion forces us to consider the strong community a locus of value in its own right.
That is, just as a unified person may be rationally constructed from appropriately related temporal parts, so too a unified community might be rationally constructed from appropriately related individuals. In either case, the value accruing to the whole may transcend the sum of its parts: something may be good for a person without being good for any particular momentary stage, or good for a community without being good for any particular individual. (But note that in either case the holistic goods will presumably be valued by the atoms, even if they are not, strictly speaking, valuable for the isolated entity alone.)
Sound plausible? (Yikes, I'm turning into a communitarian...!)