 This solution was, in outline, proposed by Richard Cartwright in his 1987 discussion of the identity theory (Cartwright 1987: 76–8; cf. David 2002: 128–9; Dodd 2008a: 86–8; Candlish &amp; Damnjanovic 2018: 265–6). According to Cartwright, the true proposition, and fact, that there are subways in Boston exists in other possible worlds where Boston does not have subways, even though in those worlds that fact would be not be a fact. (Compare: grumpy Socrates exists in worlds where Socrates is genial and sunny, but he is not grumpy there.) So even in worlds where it is not a fact that Boston has subways, that fact, namely the fact that Boston has subways, continues to exist. Cartwright embellishes his solution with two controversial points. First, he draws on Kripke’s distinction between rigid and non-rigid designation, suggesting that his solution can be described by saying that the expression “The fact that Boston has subways” is a non-rigid designator. But it is plausible that that expression goes on referring to, or being satisfied by (depending on how exactly one wants to set up the semantics of definite descriptions: see Gaskin 2008: 56–81), the fact that Boston has subways in possible worlds where Boston does not have subways; it is just that, though that fact exists in those worlds, it is not a fact there. But that upshot does not appear to derogate from the rigidity of the expression in question. Secondly, Cartwright allows for a true reading of “The fact that there are subways in Boston might not have been the fact that there are subways in Boston”. But it is arguable that we should say that this sentence is just false (David 2002: 129). The fact that there are subways in Boston would still have gone on being the same fact in worlds where Boston has no subways, namely the fact that there are subways in Boston; it is just that in those worlds this fact would not have been a fact. You might say: in that world the fact that there are subways in Boston would not be correctly described as a fact, but in talking about that world we are talking about it from the point of view of our world, and in our world it is a fact. (Similarly with grumpy Socrates.)