With the background provided by the previous chapters, the dyadic predicate “water” is discussed in some detail in this chapter as illustrative of substance predicates. The familiar claim “Water is H \(_{2}\) O” is a purely macroscopic claim about what chemists call the composition of water. Relating the macroscopic term “water” to microscopic constitution must take cognizance of the fact that the situation is a dynamic one, not a fixed, unchanging structure. The continual restructuring renders macroscopic quantities effectively homogeneous for macroscopic times, say of the order of a millionth of a second or longer. (Spectroscopists describe the structure accessible over shorter times via higher frequency radiation as indiscernible—blurred—over the longer times probed by lower frequency radiation.) What should we say about extremely small parts of a quantity of water for extremely short subintervals of a time? Infinite divisibility of time entails that intervals have proper subintervals without limit. A uniform treatment of quantities is less obviously appropriate in moving from the macro- to the microdomain of quantum mechanically indiscernible entities. Nevertheless, a uniform treatment is considered, bearing in mind that the parts are parts of a macroscopic quantity for the times at issue and not isolated from the bulk matter. But an alternative treatment is also developed where, following French and Krause’s two-sorted interpretation in which identity is meaningfully predicated of macro- but not microentities, the mereology of quantities is restricted to exclude microentities as parts. This is consistent with the conclusion that might be drawn from the discussion in Chap. 4 , that the quantity, time and space variables to which predicates at issue here are affixed range over macroscopic domains. Phase predicates don’t follow the same pattern, mereologically speaking, as substance predicates. The microscopic understanding of the dynamic equilibrium between the exchange of matter between abutting phases calls for a different mereological interpretation and an accumulation condition similar to that applying to the occupies relation comes into play. A final section addresses some of the points raised in the discussion of natural kinds in the literature.