Reciprocity

The last critical conceptual idea in our Newtonian theoretical framework for thinking about motion is reciprocity.  By this we mean to emphasize the idea that an interaction is about two objects, not just one.  So when the hammer hits the bowling ball and makes it move forward, the ball also hits the hammer and makes the hammer slow down (or even stop or change the direction of its motion).  This tells us:

Whenever two objects interact, they exert forces on each other.  Whenever we have a force $\overrightarrow{F}_{B\rightarrow A}$ of an object B acting on object A, there is also a force $\overrightarrow{F}_{A\rightarrow B}$ from object A acting on object B.

While this seems fairly trivial — after all, you can't touch something without being touched back — this principle has profound consequences, once we quantify it in Newton's third law.