  As a subject, an observer is fundamentally at rest in a reference frame, and
this being at rest allows for measuring the motion of physical objects.
Exploring the experience of centrifugal force brings to the fore the natural
inclination to see oneself at rest in a reference frame. The discrepancy
between the experience of centrifugal force and its lack of fundamental
significance in Newton's laws of motion points toward the importance of the
observer as a subject in his or her own reference frame, specifically the
observer's experience of being at rest in this reference frame. In addition,
the observer's being at rest for himself or herself in a reference frame is a
central feature of both special and general relativity.
