Einstein searched for unifying principles, from special and general theories of relativity to a unitary field theory, still incomplete. Several alternative approaches to a unitary theory suggest the difficulty of this problem. Page 9.
Bergmann, Peter G.
"The Quest for Unity: General Relativity and Unitary Field Theories,"
Syracuse Scholar (1979-1991): Vol. 1
, Article 4.
Available at: http://surface.syr.edu/suscholar/vol1/iss1/4