 For example, a typical realist about length measurement would argue that the empirical regularities displayed by individual objects’ lengths when they are ordered and concatenated are best explained by assuming that length is an objective property that has an extensive structure (Swoyer 1987: 271–4). That is, relations among lengths such as “longer than” and “sum of” exist independently of whether any objects happen to be ordered and concatenated by humans, and indeed independently of whether objects of some particular length happen to exist at all. The existence of an extensive property structure means that lengths share much of their structure with the positive real numbers, and this explains the usefulness of the positive reals in representing lengths. Moreover, if measurable properties are analyzed in dispositional terms, it becomes easy to explain why some measuring instruments are reliable. For example, if one assumes that a certain amount of electric current in a wire entails a disposition to deflect an ammeter needle by a certain angle, it follows that the ammeter’s indications counterfactually depend on the amount of electric current in the wire, and therefore that the ammeter is reliable (Trout 1998: 65).