In Misner, Thorne and Wheeler's "Gravitation" chapter 39 section 3, they write the following:
"In general relativity theory, the metric is generated directly by the stress-energy of matter and of nongravitational fields".
My question is then if stress-energy is responsible for the generation of the metric, to what extent can the metric be defined prior to introducing stress energy? Furthermore, the Einstein equations can be derived from an action principle whereby the stress energy tensor is defined according to the variation of the matter action with respect to the metric and so to me it seems more as though the metric works to generate the stress-energy content, as opposed to the other way around. This however feels nonsensical, as stress-energy is responsible for curvature, and the metric itself captures this.
However, one can consider vacuum solutions to the Einstein equations whereby the metric (e.g. Schwartzschild) is naturally defined without the need for stress-energy. So what is the content of Misner, Thorne and Wheeler's statement?