Ebb and flow is fundamental.  Water flows in; water flows out.

It is a basic dynamic of nature.  It is the essential pattern in describing the most simple of systems.

Complex System Design theory is the study of how systems are designed in nature and elsewhere.  The idea is to understand the patterns that underlie systems and eventually discover some sort of universal, divine truth.

The Theory of Everything would be a theory that unifies all theorems of the classical and quantum realms.  Attaining such would likely be akin to achieving nirvana.

A bathtub is a repository, typically porcelain, for storing water in while a human, or other mammal, bathes.  It does not require plumbing that allows for the input of water as historically, they were filled via water buckets.  Still, it does largely require a valve for the removal of water as, at such size, it is not manageable to expel water by lifting it and pouring said water out the window.  For what it's worth, this probably did happen in pre-gunpowder eras. 

The point is that the most basic system is defined by it's input and output.  Efficiency is how well it suffices it's intended purpose given like inputs and outputs.  Most systems are more complicated.

They say a river is never the same as it was the moment before.  A river is constant change.  It's a valuable way to understand existence against time - wherein time itself is a river.

Nothing exists absent time.  No one steps in the same river twice.

To maintain maximum power in a system, it may be necessary to control the flow of some entity from one part to another.  Ebb and flow is fundamental.