 There’s an intuitive mereology of issues, topics, or subject matters: they should be capable of overlapping and of fusing into wholes which inherit the proper features from the parts (Yablo 2014: Section 3.2). The topic philosophy and the topic mathematics overlap (the overlap including, presumably, logic). How things went in 1882 is included in the larger how things went in the nineteenth century. A larger subject matter induces smaller cells: \(w_1\) and \(w_2\) agree on what happened in their whole nineteenth century only if they agree on what happened in their 1882 to begin with.