 As in  section 4.1,  we let \(\sim\) be an equivalence relation on the set \(\mathcal{F}\) of propositions to which our agent assigns opinions. It is the relation of relevant similarity between two propositions. In  section 4.1,  we said that we would impose conditions \(C(\sim)\) on this equivalence relation, but we said no more to identify those conditions. In this section, we in fact define this equivalence relation. We take it to be relative to a credence function \(c\), so we write it \(\sim_c\), and we define it as follows: \(A \sim_c B\) iff \(c(A) = c(B)\). That is, two propositions are relevantly similar for our agent with credence function \(c\) if \(c\) assigns them the same credence. Thus, given a possible world \(w\), we say that a credence function \(c\) is perfectly calibrated at \(w\) if, for each \(A\) in \(\mathcal{F}\),   \[c(A) = \mathrm{Freq}(\mathcal{F}, A, \sim_c, w)\]