 Kant’s basic claim is not prima facie implausible—“all interest is ultimately practical and even that of speculative reason is only conditional and is complete in practical use alone” (5:121). But what he means by this, exactly, is a difficult matter of interpretation. (Cf. Neiman 1994: Ch. 3; Guyer 1997; Rauscher 1998.) Moreover, the uses to which Kant puts this argument are as controversial as any question in his philosophy, since he here reinstates—as items of faith rather than knowledge—the very ideas that the first Critique had argued to lie beyond human insight. (See further the entry on  Kant’s philosophy of religion.)