There is an element of the Oxford lecture that we have not yet addressed in much detail: Einstein's claim that “the grand object of all theory” is to unify – to make the fundamental concepts and laws “as simple and as few in number as possible.” Likewise, he had told Cornelius Lanczos in 1938 that “the physically true is logically simple, that is, it has unity in its foundation” – even if the reverse does not necessarily hold (“the logically simple does not, of course, have to be physically true.”)
Unification had always been an important aspect of Einstein's work – we need only to think of the general theory of relativity, where he unified gravitation with special relativity – and it played a prominent role in his epistemology. Some ten papers by him carried the word “Einheitliche” in their title. These were all papers on unified field theories: theories that attempted to unify in a single mathematical, preferably geometrical scheme the gravitational and electromagnetic fields. We wish to outline the role of unification in Einstein's philosophy here, followed by a brief introduction to the unified field theory program.
Unification: motivation and implementation
In his autobiography Einstein gave two criteria for a successful theory: firstly, of course, it should not contradict empirical facts. Secondly, the theory should display “inner perfection,” which was characterized by its “naturalness,” usually mathematically construed, and “logical simplicity.”