Includes letters to Speigelberg from: P. Ansgar Paus, OSB Gerleve, Germany(?); Sir K. R. Popper, The London School of Economics and Political Science, University of London; Richard H. Popkin, Editor, Journal of the History of Philosophy, Department of Philosophy, University of California, San Diego; Richard Palmer, Director of Humanities, MacMurray College, Jacksonville, Illinois; Ivor Pieydan Pearce, Department of Philosophy, University of Bristol; Ralph Barton Perry, Department of Philosophy, Harvard University, Cambridge, Massachusetts; Guenther Patzig, Philosophisches Seminar, der Universitat Gottingen; Frederick R. Lachman, Executive Editor, Encyclopedia Judaica, New York. Includes letters from Speigelberg to: Sir K. R. Popper, The London School of Economics and Political Science, University of London; Curt Peters Francis Parker, Secretary-Treasurer, The Metaphysical Society of America, Haverford College