Kant says that it can be rational to accept propositions on the basis of non-epistemic or broadly practical considerations, even if those propositions include “transcendental ideas” of supersensible objects. He also worries, however, about how such ideas (of freedom, the soul, noumenal grounds, God, the kingdom of ends, and things-in-themselves generally) acquire genuine positive content in the absence of an appropriate connection to intuitional experience. How can we be sure that the ideas are not empty “thought-entities (Gedankendinge)”—that is, speculative fancies that do not and perhaps even cannot have referents in reality? In this paper I argue for an account of the fundamental problem here (i.e. that it is based in a concern about whether or not the objects of such ideas are "really possible" in Kant's technical sense). I then critically evaluate Kant's three proposed solutions to the problem. The third one involves an appeal to the complicated doctrine of "symbolization" that becomes prominent in Kant's later works.