Levison, Arnold B.
Pegasus, 1974
Buku Teks SO
Wisdom, J. O.
Avebury, 1987
Buku Teks
Rudner, Richard S.
Prentice-Hall, 1966
Buku Teks
Morkuniene, Jurate
Council for Research in Values and Philosophy, 2004
Buku Teks
Benton, Ted
Palgrave Macmillan, 2011
Buku Teks SO