 Because of the rigid designation of the names it contains, ‘Hesperus = Phosphorus’ is necessarily true. Since empirical work on the part of astronomers was required to determine that the statement is true, we seem to have an illustration of necessary, a posteriori truth. Thus, rigid designation seems to play a crucial role in securing necessity with surprising epistemological significance. Prior to discussion about rigid designation, the necessary a posteriori was generally thought to be an empty category. Rigid designation, along with other referential mechanisms discussed in connection with it, has changed all that, going “against the assumption of centuries of philosophers,” as Linda Zagzebski relates (Zagzebski 2017, p. 213).  Thus, Putnam observes that rigidity has “startling consequences for the theory of necessary truth” (1975, p. 232).