Paula Modersohn-Becker, Günter Busch, Liselotte von Reinken, Arthur S. Wensinger, Carole Clew Hoey (1998). “Paula Modersohn-Becker, the Letters and Journals”, p.121, Northwestern University Press
William Batchelder Greene (1872). “The blazing star; with an appendix treating of the Jewish Kabbala, also a tract on the philosophy of H. Spencer, and one on New-England transcendentalism”, p.3
Ichabod Smith Spencer (1856). “A pastor's sketches: or, Conversations with anxious inquirers respecting the way of salvation. With an intr. and ed. notes, by J.A. James”, p.23
President Barack Obama's remarks at Clinton Global Initiative at Sheraton New York Hotel and Towers in New York City, obamawhitehouse.archives.gov. September 23, 2014.