 The puzzle doesn’t depend on the credentials of (20) as an identity sentence. Even if, for whatever reason, it isn’t, it is still hard to see how truth-value can change going from (21a) to (21b), granted that the proposition that Moriarty has returned and that Moriarty has returned are co-denoting. There also doesn’t seem to be a useful application of a traditional account of referential opacity, such as Frege’s (discussed in connection with (2c) above). Substitution-failure is explained in Fregean terms by co-denoting expressions having different senses, but it’s unclear that adding or deleting ‘the proposition’ is a significant enough difference to change sense. We also expect that examples of substitution-failure become unrealistic if the subject is attributed explicit belief in the identity premise. However, adding that Holmes is absolutely clear about the truth of (20) and has it at the forefront of his consciousness doesn’t make it any more likely that (21b) is true, even though (21a) is. 