Einstein (I think) in both SR & GR developed the concept of an event specified as occurring at space-time coordinates (x, y, z, t). This leads to the concept of World Lines: A set of (x, y, z, t) coordinates which describe the path of a particle. It might have been Minkowski rather than Einstein who developed the concept of World Lines & the notion of viewing Einstein's theories as Metric Geometery in a 4D Space-Time continuum. The 4D Space-time continuum is often referred to as Minkowski Space, indicating that the concept is probably his & adopted as useful by Einstein. The concept provides answers to some philosophical arguments which lasted for centuries. One such argument relates to the concept of the continued existence of an object, either animate or inanimate. What does it mean to claim that an object viewed today is the same object it was yesterday? Note that it is obvious that any object gains or loses atoms/molecules from day to day. It is obvious that the atoms/molecules comprising an object change their configuration from day to day. If an object is defined as a set of related World Lines, the philosophical argument is resolved. An atom/molecule which moves some distance from the object is no longer a member of the set. Its World Lines are no longer related to the other World Lines in the set. An atom/molecule which moves from some distance & becomes part of the object becomes a member of the set: Its World Line is now a member of the set. Atoms/molecules which change their configuation are still part of the set. The above model describes a 4D Space-Time continuum which is static: There is no motion. Whether a person likes or dislikes the notion of a static universe, the model is very useful. Those who are comfortable with the notion of a motionless 4D universe can accept the model as is. Those uncomfortable with the notion can make use of the model & reject the notion of a motionless universe. They take the reasonable POV that a model is only a model, it is not reality.