Rudner, Richard S.
Prentice-Hall, 1966
Buku Teks
Benton, Ted, author
Palgrave Macmillan, 2011
Buku Teks
Ryan, Alan
Oxford , 1970
Buku Teks
Morkuniene, Jurate
Council for Research in Values and Philosophy, 2004
Buku Teks
edited by Richard Boyd, Philip Gasper, and J. D. Trout
Bradford Book, 1997
Buku Teks