 The mass-count distinction displays, in a particularly striking way, a discrepancy between the ontology displayed by natural language and the ontology tied to cognition or what is generally taken to be the ontology of the real. Various criteria distinguish count nouns from mass nouns in English. Most important is the availability of the plural and the applicability of cardinal and ordinal numerals with count nouns, but not mass nouns (Pelletier &amp; Schubert 1989/ 2003; Doetjes 2012). The mass-count distinction, it is generally agreed, has semantic content, but there is much less agreement as to what that content amounts to (see entry on  metaphysics of mass expressions).  The mass-count distinction has often been taken to reflect the ontological distinction between individuals (chair, door) and matter or stuff (metal, wood), a distinction that has been cast either in terms of the notion of having a boundary or integrity or being an atom (with respect to some concept). However, more recently linguists have drawn attention to a number of generalizations that indicate that the distinction between singular count, mass, and plural nouns does not strictly go along with an ontological distinction between different types of beings (Pelletier &amp; Schubert 1989/2003; Chierchia 1998; Moltmann 1997; Rothstein 2017). First, singular count NPs seem to be able to stand for the very same things as definite plural or mass NPs (the (loose) collection of stuff on this desk—the stuff on this desk, the quantity of liquid in the container—the liquid in the container). Second, languages may make a choice of mass as opposed to count without apparent perceptual differences (rice—oats, corn—peas, cattle—cows). Also cross-linguistically, the choice of mass versus count for particular entities appears to an extent arbitrary (English hair—Italian capelli, English pasta—French pâtes). Third, there is an important class of so-called object mass nouns, mass nouns that appear to stand for pluralities of well-individuated objects, such as hardware, jewelry, luggage, staff, police force, often competing with apparent co-extensional plural nouns in the same language (clothes—clothing, policemen—police force, cows—cattle, carpets—carpeting). Yet like other mass nouns, object mass nouns resist numerals (* the three police force) as well as a range of other predicates applicable to plurals (John listed the clothes / ??? the clothing, The policemen / ?? the police force are / is numerous). Whether a language chooses a singular count, plural, or mass noun appears to an extent arbitrary; yet the choice between mass and count plays a role for the applicability of number-related predicates and quantifiers. The mass-count distinction thus displays a form of “grammaticized individuation” (Rothstein 2017), involving a language-driven notion of unity that may diverge from the notion of unity that pertains to cognition or what is taken to be the real structure of things.