  General relativity makes it hard to view the metric field simply as part of the containing spacetime. For, in addition to spatial and temporal information, the metric field also represents the gravitational field. Therefore it also carries energy and momentum—the energy and momentum of the gravitational field (although a notorious technical problem in general relativity precludes identifying the energy and momentum density of the gravitational field at any particular event in spacetime). This energy and momentum is freely interchanged with other matter fields in spacetimes. It is the source of the huge quantities of energy released as radiation and heat in stellar collapse, for example. To carry energy and momentum is a natural distinguishing characteristic of matter contained within spacetime. So the metric field of general relativity seems to defy easy characterization. We would like it to be exclusively part of spacetime the container, or exclusively part of matter the contained. Yet it seems to be part of both.