(taken from Lewis 1968): A1. (Nothing is in anything except a world) A2. (Nothing is in two worlds) A3. (Whatever is a counterpart is in a world) A4. (Whatever has a counterpart is in a world) A5. (Nothing is a counterpart of anything else in its world) A6. (Anything in a world is a counterpart of itself) A7. (Some world contains all and only actual things) A8. (Something is actual) It is an uncontroversial assumption to assume that the primitives and the axioms A1 through A8 make the standard counterpart system. Comments on the axioms A1 excludes individuals that exist in no world at all. The way an individual is in a world is by being a part of that world, so the basic relation is mereological. A2 excludes individuals that exist in more than one possible world. But because David Lewis accepts the existence of arbitrary mereological sums there are individuals that exist in several possible worlds, but they are not possible individuals because none of them have the property of being actual. And that is because it is not possible for such a whole to be actual. A3 and A4 make counterparts worldbound, excluding an individual that has a non-worldbound counterpart. A5 and A6 restrict the use of the CT-relation so that it is used within a possible world when and only when it is stood in by an entity to itself. A7 and A8 make one possible world the unique actual world. Principles that are not accepted in normal CT R1. (Symmetry of the counterpart relation) R2. (Transitivity of the counterpart relation) R3. (Nothing in any world has more than one counterpart in any other world) R4. (No two things in any world have a common counterpart in any other world) R5. (For any two worlds, anything in one is a counterpart of something in the other) R6. (For any two worlds, anything in one has some counterpart in the other) Motivations for counterpart theory CT can be applied to the relationship between identical objects in different worlds or at different times. Depending on the subject, there are different reasons for accepting CT as a description of the relation between different entities. In possible worlds David Lewis defended Modal realism. This is the view that a possible world is a concrete, maximal connected spatio-temporal region. The actual world is one of the possible worlds; it is also