Sir Issac Newton is known (among other things) for defining three physical laws of motion that are the underpinning of classic mechanics.  The third law is sometimes used in behavioural analogies.

Newton’s Third Law of Motion

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

This is a law (not a theorum) because there is mathematical and experimental proof that it is correct.  In a mechanical sense, this law has numerous practical applications.

How humans behave

The behavioural application of this law ( not proven) states every action has an equal but opposite reaction.  The idea is something like, if you shout at me, you can reasonably expect me to shout back at you (with equal volume).

Conceptually this means you should expect a contrary reaction when you take an action.  For best results, your behavioural strategies should include how to resolve (or at least mitigate) these reactions.

Not Universal

I don’t believe every action I take has an equal and opposite reaction.  That is, outside the laws of physics (e.g. the behavioural space) this law is not universal in it’s applicability.

It does however resonate to a certain level of logic and we should never be surprised when are actions do create a reaction.  This is something it is better to be prepared for, than not.  (Boy scouts will get this.)