1st Law and Newtonian space and time. One of the most important consequences of the First Law is that it defines what we mean by an inertial frame of reference. An inertial reference frame is a reference frame where isolated bodies are seen to move in straight lines at constant velocity. An observer at rest with respect to an inertial frame of reference is called an inertial observer . For example, an inertial observer will find that a body on which no forces act moves in a straight line at constant speed or is at rest.
All motion occurs in space and is measured by time.
In Newton's model both space and time are unaffected by the presence or absence of objects. That is space and time are absolute , an arena where the play of Nature unfolds. In Newton's words, Absolute space in its own nature, without relation to anything external, remains always similar and immovable. A consequence of this is that a given distance will be agreed upon by any two observers at rest with respect to each other or in uniform relative motion, for, after all, they are just measuring the separation between two immovable points in eternal space. In the same way a time interval will be agreed upon by any two observers for they are just marking two notches on eternal time.