  Kant's Third Law of Mechanics, expressed in Proposition 4, asserts the equality of action and reaction in the communication of motion.  Kant formulates a version of the Third Analogy of Experience (according to which all external action in the world is interaction) and suggests that the main point at issue in mechanics is establishing that mutual action is necessarily reaction. Kant's argument for this law is based on the following line of thought: (i) if all changes of matter are changes of motion; (ii) if all changes of motion are reciprocal and equal (since one body cannot move closer to/farther away from another body without the second body moving closer to/farther away from the first body and by exactly the same amount); and (iii) if every change of matter has an external cause (a proposition that was established as the Second Law of Mechanics), then the cause of the change of motion of the one body entails an equal and opposite cause of a change of motion of the other body or, in short, action must be equal to reaction.