So if your evidence constrains your belief in \(X\) and in \(Y\), but is silent on their interaction, then you will only be able to pin down these compound events to certain intervals. Any choice of a particular probability function will go beyond the evidence in assuming some particular evidential relationship between \(X\) and \(Y\). That is, \(p(X)\) and \(p(X\mid Y)\) will differ in a way that has no grounding in your evidence.