A centerpiece of our observation theory is the probabilistic connection between theory and observation that enables one to derive observational consequences from theories about the distribution of observer-moments in the world. Here we shall first propose an equation that gives a specification of this fundamental methodological link, and then we shall illustrate how it works by applying it to Incubator.
Let be an observer-moment whose subjective probability function is . Let be the class of all possible observer-moments that belong to the same reference class as (according ’s reference class definition ).87 Let be the possible world in which is located. Let e be some evidence and hsome hypothesis, and let and be the classes of possible observer-moments “about whom” e and hare true, respectively. (If h ascribes a property to observer-moments – e.g. h:= “This is an observer-moment that has a black beard” – then we say that h is true about those and only those possible observer-moments that have the property in question; if h is non-indexical, not referring to any particular observer-moment, then h is true about all and only those possible observer-moments that live in possible worlds where h holds true. And similarly for e.) Finally, let be the class of observer-moments in the possible world w. We then have:
be an observer-moment whose subjective probability function is 
. Let 
be the class of all possible observer-moments that belong to 
).87 Let 
be 
and 
be the classes of possible observer-moments “about whom” e and h 
be the class of observer-moments in the possible world w. We then have: