 One puzzling feature of McTaggart’s mereological system is his apparent acceptance of a form of relative identity: x and y might be the same substance while simultaneously being different groups. In section 128 of the first volume of the Nature of Existence, McTaggart considers the group consisting of the counties of Great Britain and the group consisting of the parishes of Great Britain. McTaggart argues that although these are not the same groups, we ought to say that they are the same substance. He does not appear to merely mean that this substance may be partitioned in two different ways, and that corresponding to these ways are two numerically distinct groups.