 In his classic work Der Logische Aufbau der Welt (1928) (known as the Aufbau and translated as The Logical Structure of the World), Carnap investigated the logical ‘construction’ of objects of inter-subjective knowledge out of the simplest starting point or basic types of fundamental entities (Russell had urged in his late solution to the problem of the external world to substitute logical constructions for inferred entities). He introduced several possible domains of objects, one of which being the psychological objects of private sense experience—analysed as ‘elementary experiences’. These provided the simplest but also the ‘natural starting point in the epistemic order of objects’. This specific construction captures the fact that knowledge is someone’s private knowledge, while it raises Carnap’s (and Kant’s) main problem, namely, how then objective knowledge is possible. In the Aufbau and especially in Pseudoproblems in Philosophy, also here the inclusion of terms and statements in the system of science by this method was intended to exclude all nonsensical speculative statements. It provided a verification criterion not only of testing but also of cognitive significance, or meaning. Carnap stated in that vein that ‘science is a system of statements based on direct experience’ (Carnap 1932/1934).