The pragmatic ambiguity (or better pragmatic ambivalence) of deontic sentences is a very well-known phenomenon. This phenomenon has been isolated by the German logician Christoph Sigwart (1830–1905). Deontic sentences (ought-sentences) are subject to a double interpretation: they may be used to prescribe norms (prescriptive deontic sentences) or they may be used to describe norms (descriptive deontic sentences).¹