O`Brien, Dan
Polity Press, 2006
Buku Teks
Lewis, Clarence Irving
Dover, 1956
Buku Teks
Chisholm, Roderick M.
Prentice Hall International, 1989
Buku Teks
Rescher, Nicholas
State University of New York Press, 2003
Buku Teks
Wojciechowski, Jerzy A.
Council for research in values and philosophy, [Date of publication not identified]
Buku Teks SO