Abstract
The International Vocabulary of Metrology (VIM) is a foundational document of measurement science, with the ambition of providing a system of "basic and general concepts and associated terms" of metrology. Such a system has evolved with the evolution of measurement science, and this is particularly manifest in the case of the cluster of concepts around 'quantity'. The present paper presents some aspects of this development, by first remarking on the importance of terminology for measurement science and then introducing the basic problem what is a quantity? as a way to provide a well-grounded explanation to the usual claims that two quantities can be equal, or two values of quantities can be equal, or a quantity of an object can be equal to a value of a quantity. The analysis develops along three questions: (1) what is the relation between general properties and individual properties? (2) what is the relation between properties and quantities? (3) what is the relation between quantities of objects and values of quantities?