Lewes, George Henry, author
Cambridge University Network, 2012
eBooks
Mayer, Frederick, author
American Book Company, 1951
Buku Klasik
edited by John K. Ryan
The Catholic University of America Press, 1963
Buku Teks
Kenny, Anthony, author
Blackwell Publishing, 2007
Buku Teks
Dray, William H.
Prentice-Hall, 1993
Buku Teks