Einstein stated that a theory’s truth can never be proved, as future experience may contradict it. A theory can only be shown incorrect, through logical failure or contradiction by a fact. Intuition alone can decide between two competing theories agreeing with the facts.

Nobody who has really gone deeply into the matter will deny that in practice the world of phenomena uniquely determines the theoretical system, in spite of the fact that there is no logical bridge between phenomena and their theoretical principles; this is what Leibniz described so happily as a ‘pre-established harmony.’