The Stone Pulls the Horse

Newton's Third Law We think of this as F = F, or "for every action there is an equal and opposite reaction", but Newton expressed this differently than the way we learned this in school.   In his work Philosophiæ Naturalis Principia Mathematica, first published in 1687, there were no equations in his laws, and they were grounded in concrete reality.  He added a physical description one could envision in the mind, rather than the abstract way we think of it:

If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone, as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other. [Newton, 1687]

Why didn't my teacher ask me how the little stone can pull a horse with as much force as the horse pulls the stone?   "It is the tension in the rope!" would have been something fun to discover in high school.  Instead, I just read "action = reaction".

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