Occurrents, Classes, and Masses

Abstract This chapter discusses the mereology of three kinds of object for which it is reasonable to suppose the extensionality principle PPP holds: occurrents, classes, and masses. Classes and masses provide the most plausible concrete interpretations of classical extensional meteology. The ontological categories of class and mass have been relatively neglected by comparison with that of individuals, and they are introduced in the present chapter. The concept of class is not that found in contemporary set theory, but what is argued to be the old-fashioned notion of a plurality of individuals; this view is also explained in this chapter. Activities and performances are also discussed, along with plural reference, pluralities, analogies between ontology and mereology, mass reference and masses, parts of groups, and further possible applications of extensional mereology.