Kusch, Martin, author
Routledge, 1999
Buku Teks
Pratt Vernon
Methuen Press, 1978
Buku Teks
Rudner, Richard S.
Prentice-Hall, 1966
Buku Teks
Ryan, Alan
Oxford , 1970
Buku Teks
Morkuniene, Jurate
Council for Research in Values and Philosophy, 2004
Buku Teks