Einstein on the earmarks of a good theory:

"[Einstein] once told Werner Heisenberg, in the early years, I may have thought that you can deduce theories from experiments, but it's nonsense. On the contrary, he says, theory tells you what it is you should be observing. Theory to him was basic. He once wrote in his autobiographical notes about the earmarks of a good theory. There are two. One is that it should not be falsified or disproved by experiments. It doesn't even have to be proved by experiments as long as it doesn't contradict experience. And, secondly, it should have a kind of persuasive rationality, a kind of internal beauty that makes you convinced that you're on the right track - an aesthetic criterion."

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