Medieval geographers deduced a Nile through Kanem from climate theory, not observation

Evidence

On the one hand, they were caused by insufficiency of empirical knowledge about the real world; on the other hand, they were caused by a fixed scheme of categories, according to which the world was divided into “temperate” and “extreme” climates; one of the “extreme” climates was the torrid zone, in which life was only possible along the banks of great rivers; it follows, logically, that (a) if there was agriculture and urban life in Kanem, and (b) if Kanem indeed was situated in the torrid climate, then it was to be expected that there was a “Nile” flowing across Kanem. — Eve M. Troutt Powell, A Different Shade of Colonialism_…, 144

Categorization

• XREF: Shows how deductive frameworks manufactured ‘knowledge’ about the Sudan; parallels how colonial-era racial and civilizational categories worked similarly.