[Metric]

A quote from Rovelli's "Quantum Gravity":

"In newtonian and special-relativistic physics, if we take away the dynamical entities -- particles and fields -- what remains is space and time. In general-relativistic physics, if we take way the dynamical entities, nothing remains. The space and time of Newton and Minkowski are re-interpreted as a configuration of one of the fields, the gravitational field. This implies that physical entities -- particles and fields -- are not immersed in space, and moving in time. The do not live on spacetime. They live, so to say, on one another.

It is as if we had observed in the ocean many animals living on an island: animals on the island. Then we discover that the island itself is in fact a great whale. So the animals are no longer on the island, just animals on animals. Similarly, the Universe is not made up of fields on spacetime; it is made up of fields on fields."