||The mass / count dichotomy is ill-defined and contested, having been first introduced by Jespersen as both a semantic and, at one and the same time, an ontic contrast. Many writers have noted that these two contrasts in fact diverge substantially, and have often emphasized one contrast at the expense of the other. However, if and when intended as a mutually exclusive and (virtually) exhaustive contrast of two types of nouns, or occurrences or uses of nouns, the dichotomy is properly conceived as semantic, and is equivalent to the dichotomy of count and non-count nouns. The question then arises as to just what the semantic analyses of these two classes might involve. And here, among philosophers and linguists alike, the standard view posits a contrast between count nouns, as capable both of semantically singular and plural occurrences, and mass nouns, as capable of singular occurrences exclusively. Nevertheless, this view is contradicted by Jespersen’s assertion that ‘Mass-words are totally different, logically they are neither singular nor plural, because what they stand for is not countable’. And an account of precisely this non-standard form has been recently defended in detail by Laycock, and also endorsed by others. Moving beyond the semantic question now, metaphysical interest is generally directed to a specific subset of concrete mass nouns – presumed 'core' mass nouns such as 'sugar', 'gold' and 'water' that figure as words for kinds of material stuff. Within the framework of the standard semantically singularist account, these nouns are held to denote individual ‘parcels’ or ‘quantities’ of stuff and subjected to various mereological treatments. The question then has to be addressed of what to do about a wide range of ‘collective’ concrete non-count nouns, such as 'furniture' and 'footwear', which typically range, not over materials or kinds of stuff, but over individual objects referred to en masse. Again, and entirely distinct from concrete mass nouns, there are the abstract non-count nouns (not always classified as ‘mass’) which include such nouns as ‘tension’, ‘sorrow’, and ‘mercy’. These nouns have received less attention, and are commonly deployed metaphorically, as in ‘The tension gradually evaporated’, ‘Their hearts were filled with sorrow’, and the quality of mercy ‘falleth as the gentle rain from heaven upon the place beneath’. But in all of this, the purely semantic (including model-theoretic) issues are common ground for linguists and philosophers alike, while the narrower range of metaphysical issues (see now the entry under Stuff) remain the preserve of philosophy.