  But notice that a specification of possible metaphysical unities alone will not be sufficient, for what we need in addition is a specification of which such unities are significant for ethics. To see why this is an issue, consider just reductionism. Reductionism is actually quite a general metaphysical view, holding at its most basic that the facts about identity simply consist in more particular facts about brains, bodies, and so forth. But even if one accepts reductionism, and so abandons appeal to some further fact about separately existing entities to explain personal identity, and even if one also believes that identity is not what matters in survival, one still has much work to do before being able to apply the theory to ethics. This is because there are (at least) four possible metaphysical units that could be targeted for normative theorizing. First, we might target living human beings, human organisms picked out by the Biological Criterion (and to some extent the Anthropological View) and unified over time via biological continuity. These entities would endure from some early-stage fetuses until organismic death. Second, we might target Lockean persons, entities picked out by the Psychological Criterion and unified over time by psychological continuity (overlapping chains of strong psychological connectedness). These entities would endure from late infancy (or the time at which the various psychological connections could be established) to brain death or perhaps dementia. Third, we might target selves, entities unified by strong psychological connectedness. Such units would have significant duration, but they would not be likely to endure for as long as persons – insofar as memories typically fade, beliefs and desires are lost or revised over time, and so forth – and they certainly wouldn't endure as long as the life of the human being of which they were a part. Fourth, we might target atoms, or momentary experiencers, “units” defined and delimited by the duration of an experience. It could be, after all, that if the deep fact of identity is missing, there just are no other relations of significance we could legitimately substitute for it, so all that remains would be merely the basic atomic moments of people's lives. (For discussion of the last three possible units, see D. Shoemaker 1999, 401; for a similar distinction, see Brink 1997b, 110–115, where he labels these last three possible units “persons,” “person-segments,” and “person-slices.”)