Savi, Julio, author
Royal Falcon Books, 2012
Buku Teks
Giordan, Giuseppe, editor
Springer, 2011
eBooks
Hick, John H., 1922-2012, author
Prentice-Hall, 1990
Buku Teks
Pojman, Louis P.
English : 6th ed., International ed., 2012
Buku Teks
Clack, Brian R., author
Polity Press, 2008
Buku Teks