  In fact, Lewis whole-heartedly accepts that things have accidental  properties and, indeed, would accept that   (16)  is robustly true. His explanation involves one of the most  interesting and provocative elements of his theory: the doctrine of  counterparts. Roughly, an object y in a world  w2 is a counterpart of an object x in  w1 if y resembles x and nothing else  in w2 resembles x more than   y.[19]   Each object is thus its own (not necessarily unique) counterpart in the world it inhabits but will typically differ in important ways from its other-wordly  counterparts. A typical other-worldly counterpart of Algol, for  example, might resemble her very closely up to some point in her  history — a point, say, after which she continued to live out  her life as a stray instead of being brought home by our kindly  dog-lover John. Hence, sentences making de re assertions about what Algol might have done or what she could or could  not have been are unpacked, semantically, as sentences about her  counterparts in other possible worlds. Thus, when we analyze  (16) accordingly, we have the entirely unproblematic concretist truth condition: 