CLEAN architecture

We need CLEAN architecture so that the system we built:

Independent of frameworks. The architecture does not depend on the existence of some library of feature laden software. This allows you to use such frameworks as tools, rather than having to cram your system into their limited constraints.
Testable. The business rules can be tested without the UI, Database, Web Server, or any other external element.
Independent of UI. The UI can change easily, without changing the rest of the system. A Web UI could be replaced with a console UI, for example, without changing the business rules.

The dependency rule

Source code dependencies can only point inwards
Nothing in an inner circle can know anything at all about something in an outer circle
The name of something declared in an outer circle must not be mentioned by the code in an inner circle

Entities encapsulate Enterprise wide business rules.
An entity can be an object with methods, or it can be a set of data structures and functions.
They encapsulate the most general and high-level rules.
They are the least likely to change when something external changes.

This layer contains application specific business rules. It encapsulates and implements all of the use cases of the system

A set of adapters that convert data from the format most convenient for the use cases and entities

The outermost layer is generally composed of frameworks and tools such as the Database, the Web framework, etc.