Thomas, Henry
Blakiston, 1946
Buku Teks SO
Christensen, Kit R.
McGraw-Hill, 2002
Buku Teks
edited by F.S.C. Northrop
Yale University Press, 1949
Buku Teks SO
Iannone, A. Pablo
Routledge, 2001
Buku Referensi
Reader in Philosophy, University of The Panjab, 1965
Buku Teks SO