MLA

Cassirer, Ernst, 1874-1945. Unendliche Zahl, Renouvier, Couturat, Russell, Zeitbegriff, Fichte, Nachträge (Envelope #96). Box 53, folder 1067. n.d.. https://collections.library.yale.edu/catalog/32380654.

APA, 6th edition

Cassirer, Ernst, 1874-1945. (["Box 53, folder 1067"])(n.d.)[Unendliche Zahl, Renouvier, Couturat, Russell, Zeitbegriff, Fichte, Nachträge (Envelope #96)]. https://collections.library.yale.edu/catalog/32380654.