 What makes one object a counterpart of another? According to Lewis, this is a matter of qualitative similarity in certain respects: a counterpart of Biden is an individual that sufficiently resembles the actual Biden in relevant respects, and more so than any other object in its world. Sometimes Lewis drops the second clause (e.g. Lewis 1968, 114–115), and at one point he merely suggests (without explanation) that counterparthood “usually involve[s] similarity” (Lewis 1986e, 8).