 One can understand that the conclusion follows from the premises, on the basis of one’s understanding of the concepts involved. One need not know anything about the identity of Peter, Greg’s cousin. Still, many have thought that (4) is not deductively valid, despite its credentials as truth-preserving on analytic or conceptual grounds. It is not quite as general as it could be because it is not as formal as it could be. The argument succeeds only because of the particular details of family concepts involved.