 A. Prior (1960), using the apparatus of contemporary modal logic, tried to characterize the distinction between normative and non-normative sentences in formal terms, which enabled him to define senses of ‘descriptive content’ versus ‘normative content’. A problem, however, occurs with mixed sentences, which have both descriptive and normative components, and Prior comes up with a paradox: wherever we draw the distinction between non-normative and normative sentences, unexpected inferences from non-normative premises to normative conclusions may appear by a mere use of laws of classical propositional logic. Consider, for instance, the following two inferences: