Furthermore an analogous difficulty arises over the question of distance.

The distance between two instantaneous positions, _i.e._ between two event-particles, is different in different space-systems.

What space is to be chosen?
Thus again the law lacks precise formulation, if relativity is accepted.
Our problem is to seek a fresh interpretation of the law of gravity in which these difficulties are evaded.

In the first place we must avoid the abstractions of space and time in the formulation of our fundamental ideas and must recur to the ultimate facts of nature, namely to events.
Also in order to find the ideal simplicity of expressions of the relations between events, we restrict ourselves to event-particles.

Thus the life of a material particle is its adventure amid a track of event-particles strung out as a continuous series or path in the four-dimensional space-time manifold.