In his Mathematical Principles of Natural Philosophy, Isaac Newton (1642–1727) distinguishes between absolute and relative time: “Absolute, true, and mathematical time . . . flows equably without relation to anything external, and by another name is called duration: relative, apparent, and common time, is some sensible and external (whether accurate or unequable) measure of duration by the means of motion, which is commonly used instead of true time; such as an hour, a day, a month, a year” (p. 6).

The notion of absolute time is crucial for Newtonian physics because its First Law of Motion implies that a body on which no forces act moves uniformly in a straight line at constant speed, or it is at rest. Only against the background of absolute time and space can rest and equable translation as free from external influence stand out against those deformations of motion that indicate external forces. Thus, absolute time in Newtonian physics is an a priori presupposition, and it is essential for the frame of reference, against which all forces are determined.

Newton himself considered that in reality there might exist no absolutely equable form of motion representing this absolute time: It might not be the time of a particular clock. But still, the assumed flowing of absolute time should not be liable to any change.

However, the laws of classical mechanics, which describe the motions of massive bodies, do not distinguish a direction of absolute time: No feature of the mechanical world would change, if time were reversed. Because the basic differential equations of classical mechanics are time reversal invariant, the future development of any mechanical system is in principle derivable from its past state, and vice versa. Thus, development from past to future and from future to past are physically equivalent.