Editing Path integral formulation (section)

=== As a probability ===
Strictly speaking, the only question that can be asked in physics is: ''What fraction of states satisfying condition {{mvar|A}} also satisfy condition {{mvar|B}}?'' The answer to this is a number between 0 and 1, which can be interpreted as a [[conditional probability]], written as {{math|P(''B''{{!}}''A'')}}. In terms of path integration, since {{math|P(''B''{{!}}''A'') {{=}} {{sfrac|P(''A''∩''B'')&nbsp;|&nbsp;P(''A'')}}}}, this means
: <math>\operatorname{P}(B\mid A) = \frac
{\sum_{F \subset A \cap B}\left| \int\mathcal{D}\varphi O_\text{in}[\varphi]e^{i\mathcal{S}[\varphi]}  F[\varphi]\right|^2}
{\sum_{F \subset A} \left|\int\mathcal{D}\varphi O_\text{in}[\varphi] e^{i\mathcal{S}[\varphi]} F[\varphi]\right|^2},</math>
where the functional {{math|''O''<sub>in</sub>[''ϕ'']}} is the superposition of all incoming states that could lead to the states we are interested in. In particular, this could be a state corresponding to the state of the Universe just after the [[Big Bang]], although for actual calculation this can be simplified using heuristic methods. Since this expression is a quotient of path integrals, it is naturally normalised.