Eruvin 36a and Eruvin 36b, on how we evaluate safek situations, situations of doubt, particularly when the doubt is about when a certain status changed, when a presumed status helps us with the eruv, as well as the principle of the eruv needing to have been defined before it took effect. Then, moving on to bererah and the ervu, to whether a Jew can set up one or multiple eruvin and only later have events define which or whether the eruv took effect.
