 In a way, (P.12ξ,1) would seem the obvious choice, corresponding to the idea that a sum of two objects is just a minimal upper bound of those objects relative to P (a partial ordering). However, this condition may be regarded as too weak to capture the intended notion of a mereological sum. For example, with ξ construed as overlap, (P.12ξ,1) is satisfied by the model of Figure 7, left: here z is a minimal upper bound of x and y, yet z hardly qualifies as a sum “made up” of x and y, since its parts include also a third, disjoint item w. Indeed, it is a simple fact about partial orderings that among finite models (P.12ξ,1) is equivalent to (P.11ξ), hence just as weak.