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Why are rest and constant straight-line motion treated as similar states in Newtonian mechanics?

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Title page of Isaac Newton's Philosophiae Naturalis Principia Mathematica, 1687, featuring large Latin text and publication details in a bordered layout. — Newton’s laws of motion The title page of Isaac Newton's *Philosophiae Naturalis Principia Mathematica* (1687; *Mathematical Principles of Natural Philosophy*), the work in which the physicist introduced his three laws of motion.

In Newtonian mechanics, rest and uniform motion in a straight line are treated as equivalent states because they represent the same condition of motion when viewed from different inertial reference frames.123

According to Newton’s first law of motion, a body will maintain its state of rest or constant velocity unless acted upon by an external force. In this framework, there is no fundamental physical distinction between being at rest and moving at a constant speed in a straight line; an observer moving at the same constant velocity as a particle will perceive that particle to be at rest, while another observer moving at a different constant velocity will perceive it to be in uniform motion.1453

Because the laws of physics are invariant under transformations between these inertial frames, motion with constant velocity is undetectable through physical experiments within a Newtonian system. Consequently, both states are characterized by the absence of acceleration and the absence of a net force acting on the body.643