 Realists about measurement maintain that measurement is best understood as the empirical estimation of an objective property or relation. A few clarificatory remarks are in order with respect to this characterization of measurement. First, the term “objective” is not meant to exclude mental properties or relations, which are the objects of psychological measurement. Rather, measurable properties or relations are taken to be objective inasmuch as they are independent of the beliefs and conventions of the humans performing the measurement and of the methods used for measuring. For example, a realist would argue that the ratio of the length of a given solid rod to the standard meter has an objective value regardless of whether and how it is measured. Second, the term “estimation” is used by realists to highlight the fact that measurement results are mere approximations of true values (Trout 1998: 46). Third, according to realists, measurement is aimed at obtaining knowledge about properties and relations, rather than at assigning values directly to individual objects. This is significant because observable objects (e.g., levers, chemical solutions, humans) often instantiate measurable properties and relations that are not directly observable (e.g., amount of mechanical work, more acidic than, intelligence). Knowledge claims about such properties and relations must presuppose some background theory. By shifting the emphasis from objects to properties and relations, realists highlight the theory-laden character of measurements.