The widely-accepted Grundaxiom of Karl Rahner's doctrine of the Trinity functions in contemporary theology as a means of reconciling seemingly contradictory claims. The present study, by contrast, indirectly challenges the viability of such theologies by subjecting the Grundaxiom to a thoroughgoing, immanent critique. It argues that Rahner fails to supply a credible account of how human beings learn of the existence of the immanent Trinity.