 (23) and (24) seem quite a bit like  (19)  and  (20):  these each seem to be pairs of sentences which differ in truth-value, despite differing only in the substitution of the italic expressions. So, for consistency, it seems that the Fregean should explain the apparent difference in truth-value between (23) and (24) in just the way he explains the apparent difference in truth-value between (19) and (20): by positing a difference in meaning between the italic expressions. But, first, it is hard to see how expressions like “catsup” and “ketchup” could differ in meaning; and, second, it seems that an example of this sort could be generated for any alleged pair of synonymous expressions. (A closely related series of examples is developed in much more detail in Kripke 1979.)