 Parfit, Nozick and Lewis all take psychological continuity to be integral to personal identity in the following sense. In cases where there is no branching psychological continuity is sufficient to ensure the identity of a later with an earlier person. A number of philosophers would reject this claim insisting that bodily persistence is required for personal identity. Some, for example Eric Olson, would argue that psychological continuity is irrelevant to the identity of those things, human beings, who are the only things we know to be persons. Instead, on this view, what is crucial to personal identity for humans is continuing to be the same animal (Olson 1997).