Each of the two things termed greatest or most desirable must be numerically one: otherwise no proof will have been given that they are the same; for it does not follow because Peloponnesians and Spartans are the bravest of the Greeks, that Peloponnesians are the same as Spartans, seeing that Peloponnesian and Spartan are not numerically one; it only follows that the one must be included under the other as Spartans are under Peloponnesians for otherwise, if the one class is not included under the other, each will be better than the other.