Something general will have to be said... about the points of view from which physical theories may be analyzed critically... The first point of view is obvious: the theory must not contradict empirical facts... The second point of view is not concerned with the relationship to the observations but with the premises of the theory itself, with what may briefly but vaguely be characterized as the `naturalness' or `logical simplicity' of the premises (the basic concepts and the relations between these)... We prize a theory more highly if, from the logical standpoint, it does not involve an arbitrary choice among theories that are equivalent and possess analogous structures... I must confess herewith that I cannot at this point, and perhaps not at all, replace these hints by more precise definitions. I believe, however, that a sharper formulation would be possible."
— Einstein, "Autobiographical Notes", originally published in Schilpp, Albert Einstein, Philosopher-Scientist, 1949, and reprinted as a separate book in 1979.