The aim of this article has been to show that our most successful theories in
physics are those that explicitly leave room for the unknown, while confining
this room sufficiently to make the theory empirically disprovable. It does
not matter whether this room is created by allowing for arbitrary forces as
Newtonian dynamics does, or by allowing for arbitrary equations of state for
matter, as General Relativity does, or for arbitrary motions of charges and
dipoles, as Maxwell's electrodynamics does. To exclude the unknown wholly as a
"unified field theory" or a "world equation" purports to do is pointless and of
no scientific significance.
Sir Hermann Bondi