 The difference between the strong and weak Kleene’s treatment of disjunction can be better appreciated by looking at the phenomenon of presupposition, which constitutes one of the most well-known linguistic motivations for the rejection of bivalence (other common motivating phenomena include the semantic paradoxes and vagueness, see the entries on  liar paradox  and  sorites paradox,  and  section 2.1.4  below). Consider Russell’s (1905) example: 