  The success of arguments for this conclusion actually depends on the specific  version of reductionism being advanced. After all, there are several possible ethically significant metaphysical units compatible with reductionism, and it turns out that the larger the unit, the less successful the argument will be. As the authors on this topic do, we will focus just on the three possible psychological units: persons, selves, and atoms. If one believes that the only relevant units are atoms (momentary experiencers) – given that in the absence of the further fact of identity one believes there just are no other unifying relations of any significance (i.e., Parfit's Extreme Claim) – then it is easy to see the complete analogy between individual lives and sets of lives: neither are unified by any significant metaphysical relations, so we could think of them both as just big collections of experiences, in which case there would seem to be no reason to apply distributive principles within either (or, alternatively, no reason not to apply such principles to both – how much weight they ought to bear in that case would remain open, however). But notice that if one adopts either of the other two psychology-based versions of reductionism, according to which either selves or persons are the basic moral units, the argument may not be as successful. If, for instance, it is psychological continuity that matters instead of the further fact of identity – and matters just as much as identity was thought to (i.e., Parfit's Moderate Claim) – then persons are the ethically significant metaphysical units, but then there remains a metaphysical distinction between persons, for psychological continuity, in the absence of interpersonal connectedness, fails to unify sets of lives in the way it does individual lives (see Jeske 1993, Brink 1997a). And the same goes for strong psychological connectedness, which would unify selves in a way rendering them metaphysically distinct from sets of lives.