 A series of notes taken during this period exhibit Carnap’s preoccupation with the question how the system of knowledge could, unlike the proposals of Diderot and d’Alembert in the Encyclopédie and those in its wake such as Comte’s and Ostwald’s (1914a, 1914b), be unified deductively, as Leibniz—and Frege—had envisaged (on Ostwald as a starting point for Carnap during this period, see Dahms 2016). What especially preoccupied the young Carnap was the challenge of giving the system empiricist underpinnings, i.e., finding deductive relations between sense perceptions and the abstract concepts of the advanced sciences. For a year or two he struggled with this problem without success, until finally, in early 1922, just before the publication of Der Raum, Carnap read Bertrand Russell’s Our Knowledge of the External World (1914). Here he found his answer—the concepts were to be derived not by analysis of experience, and deduction from the resulting elements, but by construction, using a “principle of abstraction” (Russell 1914a: 44–45; see also Pincock 2002). Experiences could be gathered into equivalence classes, and for purposes of constructing a “real” world, each such class can be regarded as a concept, defined by its extension. Immediate, momentary experience—the basis for knowledge among positivists, phenomenologists, as well as fictionalists such as the German philosopher Hans Vaihinger—need not be transcended in such a construction. The hitherto insurmountable problem of forcing the fluid character of lived experience into the straitjacket of deductive relations disappears.