For instance, a car is thought of as an engine, transmission, wheels, chassis and bodywork.
The engine is a self-contained part. I live on a narrowboat, and its engine is actually a diesel engine designed for a van.
So independent is the engine from the rest of the van that it can be used as a component in a different vehicle entirely.
The engine itself has distinct independent transferable components. Identical pistons can be used in several different types of engines, and the same is true for many other parts.
The engine parts themselves have components, like screws and washers, that are interchangeable with screws and washers in many other types of machinery.
This hierarchical principle is even more pronounced in the design of computer programs.
The history of programming, and computer systems in general, is the history of abstraction and combination.
Abstraction is the breaking of complex ideas and difficult techniques into simple reusable components which are understandable on their own.
Combination is used to make ever more complex and useful structures out of the simple pieces.
It is thought that this 'hierarchical design' is psychological. It makes things easier to understand if they can be understood piece by piece.
But the same hierarchies are evident in animals and plants.
I have a heart, which is interchangeable with other human and animal hearts.
There have been experiments where the hearts of pigs have been exchanged with the hearts of humans.
Arms, legs, eyes, lungs, fingers, fingernails, bones, skin have distinct functions, are transferable and independently understandable as pieces.
But evolution has no mind.
So this apparent hierarchy cannot be psychological.
What is its cause?
It is possible that the hierarchical design in nature is illusionary, and that we are, with our hierarchical minds, perceiving a structure that is not there.
I do not believe this.
I suspect there is a mathematical answer, that causes mindless evolutionary processes to produce hierarchical designs.
And I further suspect that that answer might explain why our minds like to make hierarchies.
But I do not know what the answer is, or even how properly to ask the question, and I do not know whether anyone has asked or tried to answer the question before.