 If the multiverse according to \(M\) is sufficiently vast and varied, life unavoidably appears somewhere in it, so the conditional prior \(P(R\mid M)\) must be \(1\) (or very close to \(1\)). If we assume that, on the assumption that there is only a single universe, it is improbable that it has the right conditions for life (see  Section 2.1  for discussion), the conditional prior \(P(R\mid U)\) must be much smaller than \(1\). This gives \(P(R\mid M) \gg P(R\mid U)\), which entails \(P(R\mid M)/P(R\mid U)\gg1\), which in turn entails a ratio of posteriors that is much larger than the ratio of the priors: \(\frac{P(M\mid R)}{P(U\mid R)}\gg\frac{P(M)}{P(U)}\). Unless we have prior reasons to dramatically prefer a single universe over the multiverse, i.e., unless \(P(U)\gg P(M)\), the ratio of the posteriors \(\frac{P^+(M)}{P^+(U)}\) will be larger than \(1\).