How did observation sentences help test empirical statements in Rudolf Carnap’s philosophy?

In Rudolf Carnap’s philosophy, observation sentences serve as the empirical foundation for testing scientific statements. In his later work, particularly in his essay "Testability and Meaning," Carnap moved away from the view that all empirical terms must be fully definable by immediate experience. Instead, he argued that empirical statements can be partially defined through "reduction sentences" and observation sentences, the truth of which can be checked by direct observation.1

These observation sentences describe publicly observable aspects of physical objects—such as the movement of a measuring instrument's needle or changes in a subject's pulse rate—and can be formulated within the vocabulary of physics. Carnap emphasized that these observations typically do not provide absolute proof or disproof of a scientific statement; rather, they offer varying degrees of "confirmation".1

By establishing this testability criterion, Carnap sought to distinguish meaningful scientific assertions from those that lack observational implications, which he categorized as empirically meaningless. This approach was central to his broader project of physicalism, which aimed to reduce all empirical science to the language of physics.1