 In the case of  (4),  the presupposition is clearly of a semantic nature, since the sentence “Someone is identical with Kepler”, which is true just if ‘Kepler’ has reference, is a logical consequence of  (4)  and (under a standard interpretation) of (5). By contrast, in the negated forms of  (6)  and  (7),  the presupposition can be canceled by context, e.g., as in