 As just explained, a theoretical term may simply be understood as an expression that refers to nonobservable entities or properties. Theoreticity, on this understanding, is the negation of observability. This explanation of theoreticity thus rests on an antecedent understanding of observability. What makes an entity or property observable? As Carnap (1966: ch. 23) has pointed out, a philosopher understands the notion of observability in a narrower sense than a physicist. For a philosopher, a property is observable if it can be ‘directly perceived by the senses’. Hence, such properties as ‘blue’, ‘hard’ and ‘colder than’ are paradigmatic examples of observable properties in the philosopher’s understanding of observability. The physicist, by contrast, would also count quantitative magnitudes that can be measured in a ‘relatively simple, direct way’ as observable. Hence, the physicist views such quantities as temperature, pressure and intensity of electric current as observable.