 In sum, a property or object is observable—in the philosopher’s sense—if it can be perceived directly, where directness of observation excludes the use of technical artifacts and inferences. Notably, Carnap (1936/37: 455; 1966: 226) admits that his explanation of the distinction is not sufficiently precise to determine a sharp line between observational and theoretical terms. He rather views the theory-observation distinction as being introduced into a ‘continuum of degrees of observability’ by choice. Prominent criticisms of the theory-observation distinction will be discussed in  Section 2.1.