Balance
Systems try and find their way to balance - what we call Equilibrium
By understanding this fundamental concept of systems we can understand many seemingly disconnected phenomena which are really just have the same underlying concept underneath.
pH balance in water which explains acids and bases. See pH Balance.
Other examples of systems finding balance and equilibrium are
As you can see this is one of the most powerful system concepts in terms of understanding systems theory.
One theory to rule them all.