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The action of the earth on the stone takes place indirectly. The

earth produces in its surrounding a gravitational field, which

acts on the stone and produces its motion of fall. As we know

from experience, the intensity of the action on a body dimishes

according to a quite definite law, as we proceed farther and

farther away from the earth. From our point of view this means:

The law governing the properties of the gravitational field in

space must be a perfectly definite one, in order correctly to

represent the diminution of gravitational action with the

distance from operative bodies. It is something like this: The

body (e.g. the earth) produces a field in its immediate

neighbourhood directly; the intensity and direction of the field

at points farther removed from the body are thence determined by

the law which governs the properties in space of the

gravitational fields themselves.