The equivalence principle was a fundamental foundation for GR, but there seems to be no similarly powerful principle of our time -- right? I don't count the anthropic principle, which seems to me not to be mathematically powerful, but should I? Perhaps we can say that there was such a thing as Planck's "principle", the equivalence of energy and frequency, momentum and wavelength, etc., which drove the development of the old quantum theory in the 1900s, but that doesn't seem to account well for developments in the new quantum theory in the 1920s. The correspondence principle was used as a powerful tool in the early days of the new quantum theory, but it seems not to have quite the same status as the principle of equivalence or Planck's principle. It seems worthwhile to point out that the principle of equivalence is commonly described as a "empirical" principle, whereas Planck's "principle" seems rather more subtle than just empirical. So, what principles are currently driving people's thinking? An additional question is: where are there empirical needs that might require a theory that is driven by a new principle? One big question seems to be dark matter and energy. Is this a signature of a new theory, or is it just dark matter and energy. Is this the body in the outer solar system that modifies the orbits of Jupiter and Saturn, or is it the body in the inner solar system that was supposed to modify the precession of the perihelion of Mercury? The standard model of particle physics has its empirical doubts and its theoretical difficulties, but am I right that nothing is so glaring as dark matter and energy?