 A variant of this approach takes the class of models of a language as a surrogate for possible states of affairs (Miller 1978a). The other utilizes a semantics of incomplete possible states like those favored by structuralist accounts of scientific theories (Kuipers 1987b, Kuipers 2019). The idea which these accounts have in common is that the distance between two propositions \(A\) and \(B\) is measured by the symmetric difference \(A\mathbin{\Delta} B\) of the two sets of possibilities: \((A - B)\cup(B - A)\). Roughly speaking, the larger the symmetric difference, the greater the distance between the two propositions. Symmetric differences might be compared qualitatively – by means of set-theoretic inclusion – or quantitatively, using some kind of probability measure. Both can be shown to have the general features of a measure of distance.