Einstein sets out three tests for a theory, and each pulls in the same direction. The premises should be few and simple. The theory should connect phenomena that had seemed unrelated. And it should apply across as wide a range of cases as possible.

What makes the combination demanding is that the first works against the other two. Assumptions can always be added to cover awkward cases, but each addition weakens the theory. The achievement lies in explaining more while assuming less.

Einstein wrote this while discussing classical thermodynamics, which he described as the only physical theory of universal content he was convinced would never be overthrown. Its two laws, simply stated, govern steam engines, chemical reactions and stars alike. The same standard applies to his own work: relativity begins from two postulates and reaches from the orbit of Mercury to the structure of the universe.