 Carnap remained as radically pragmatist about the choice among fictions to guide this ascent as Vaihinger; the choice of fictions was entirely practical and purpose-relative. (For more on Vaihinger’s account of fictions, see, e.g., Chapter 1 of Appiah 2017.) To obtain the scientific secondary world, Carnap maintained, we need only two fictions, corresponding roughly to Kant’s categories of cause and substance: a principle of induction (or uniformity of nature) and a principle of “continuity” (as Mach had called it), requiring certain clusters of perceptions, such as those grouped into a “physical object”, to remain constant, under certain conditions, while we are not perceiving them.