Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.

This sentence is Newton’s First Law of Motion (the law of inertia), formulated in the late 17th century as part of his synthesis of terrestrial and celestial mechanics. It appears in the opening definitions and axioms (“Axiomata sive Leges Motus”) of the first book of Newton’s *Philosophiae Naturalis Principia Mathematica* (first published 1687). Newton’s aim was to provide a small set of general laws from which the motions of bodies—on Earth and in the heavens—could be mathematically derived, countering older Aristotelian ideas that continuous motion requires continuous cause. The law sets the baseline: absent net external force, motion does not change.

Newton asserts that rest and straight-line uniform motion are not fundamentally different states; both persist unless an external force acts. The significance is conceptual as much as mathematical: it reframes “natural” motion as inertia, making change in motion (acceleration, turning, speeding up, slowing down) the phenomenon that demands explanation. This principle underwrites the modern idea of dynamics: forces are inferred from deviations from inertial motion. It also implies that everyday experiences of objects “coming to rest” are not evidence against inertia but consequences of unbalanced forces such as friction and air resistance.

1) “Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.”
2) “A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a net external force.”
3) “An object at rest stays at rest and an object in motion stays in motion unless acted upon by an unbalanced force.”