  We have just examined the leading contemporary theories of  personal identity (or what matters in personal identity), and we have also explored how those views might relate to ethics. But we have  thus far ignored what may be the most popular theory of identity  outside philosophy (and a view that a minority of  philosophers still accept as well). This is nonreductionism, according to  which persons exist separately and independently from their brains  and bodies, and so their lives are unified from birth to death in  virtue of that separately existing entity, what we will call a  Cartesian ego (but is most popularly thought of as a soul). And  although there is logical space available for a nonreductionism  according to which identity isn't what matters for survival  and our practical concerns, the universal view is instead the  opposite. Notice, then, that this view implies both a deep unity  within individual lives and a deep disunity between lives.  After all, if what unifies my life is a particular persisting  ego-substance, and that substance is wholly present at every stage of my life, then every temporal slice of my life is just as much a part  of me as every other, so if prudential concern is grounded in  identity, for example, I ought to be equally concerned for every part of my life. Further, given that my particular ego-substance is  distinct from every other person's particular ego-substance, my  special prudential concern justifiably ends at the boundaries of my  epidermis (or at the “boundaries” of my ego).