To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

This sentence is Newton’s Third Law of Motion, formulated in his mature synthesis of mechanics in the late 1680s. It appears in the section “Axioms, or Laws of Motion” in the first book of *Philosophiæ Naturalis Principia Mathematica* (first published 1687). Newton wrote the *Principia* amid intense scientific exchange in England (including the prompting and support of Edmond Halley) and in the context of debates over planetary motion and the nature of forces. The Third Law was crucial for treating interactions—collisions, pressure, tension, gravitation—as mutual, allowing Newton to build a coherent mathematical account of motion that could apply from terrestrial mechanics to celestial dynamics.

Newton asserts that forces arise in pairs: when one body exerts a force on another, the second simultaneously exerts a force of the same magnitude in the opposite direction on the first. The law does not say the bodies’ motions must be equal and opposite—only the forces—since different masses can accelerate differently. Its significance is foundational: it underwrites conservation principles (especially momentum in isolated systems) and makes “interaction” a two-way relation rather than a one-sided push. In Newton’s framework it also supports treating gravitational attraction as mutual, helping connect the motions of planets and satellites to the same mechanical rules governing everyday impacts and constraints.

1) “For every action, there is an equal and opposite reaction.”
2) “To every action there is always opposed an equal reaction.”
3) “The mutual actions of two bodies upon each other are always equal, and directed to contrary parts.”