The aim of this volume is to make the philosophical influences on the early Karl Rahner and his philosophical studies transparent. Its centerpiece is the extensive interpretation of the metaphysics of knowledge by Thomas Aquinas, which Rahner published in 1939 under the title "Spirit in the World." The text, revised in 1957 at Rahner's request by Johann Baptist Metz and widely disseminated in this improved form, is documented here in both versions. A lecture on "Truth in Thomas Aquinas" and an essay on the existential philosophy of Martin Heidegger, previously accessible only in French, complement the first part. The documentation of numerous reviews and statements by Rahner in the second part of this volume provides insight into the broader field of his philosophical interests. Finally, the third part makes previously unpublished materials accessible.