"Einstein was guided by a principle he had inferred from the known properties of gravitation, the principle of the equivalence of gravitational forces to inertial effects such as centrifugal force. The development of the Standard Model was guided by a principle called gauge symmetry, a generalization of the well-known property of electricity that it is only differences of voltages that matter, not voltages themselves. But we have not discovered any fundamental principle that governs M-theory. The various approximations to this theory look like string or field theories in spacetimes of different dimensionalities, but it seems probable that the fundamental theory is not to be formulated in spacetime at all. Quantum field theory is powerfully constrained by principles concerning the nature of four-dimensional spacetime that are incorporated in the special theory of relativity. How can we get the ideas we need to formulate a truly fundamental theory, when this theory is meant to describe a realm where all intuitions derived from life in spacetime become inapplicable?"