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Ever since Quine, there has been a consensus that the constructions Carnap undertook in the Aufbau do not solve "the external world problem." This paper seeks to reorient our understanding of the Aufbau not by arguing that Quine was wrong about whether Carnap solved the external world problem but by getting us to reconsider what the external world problem might have been for Carnap. It adduces evidence that one paper that influenced Carnap early on was Karl Gerhards's 1922 essay "Der mathematische Kern der Aussenweltshypothese." The paper outlines what Gerhards took the problem of the hypothesis of the external world to be (the unique simplicity or economy of the hypothesis of external objects given the course of experience), the tradition from which it arose (not Hume and Russell but rather Helmholtz and Mach), and how Gerhards attempted to consider this problem in its mathematical core (which becomes a problem of analysis situs and projective geometry). It then relates Gerhards's project to Carnap's in the crucial sections of the Aufbau where he projects the qualities of the autophyschological realm on a four-dimensional space-time manifold. Gerhards's work is brought in not to replace Russell's in the background to the Aufbau but rather to enrich our sense of the sources Carnap draws from and the problems he there entertains.

DOI: 10.1007/978-3-319-21876-2_1

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