 Hence, in e.g., ‘Man is a species and it is predicable of many’, or ‘Socrates runs and he disputes’, the relative can be replaced without loss of meaning by its antecedent. But in ‘A man runs and he disputes’, it cannot, for the supposition would change. ‘He’ supposits only for men who are running, whereas ‘a man’, if it replaced it, would supposit for all men.