Why is it sometimes considered that abstraction is not part of the main principles of object-oriented programming?

Abstraction is often considered not as a separate core principle of object-oriented programming (OOP), but rather as a concept underlying other principles. The main principles of OOP are traditionally identified as:

• Inheritance
• Encapsulation
• Polymorphism

Abstraction, on the other hand, is the process of highlighting the essential characteristics of an object and hiding implementation details. It helps create interfaces and simplifies interaction with objects, but by itself is not always distinguished as a separate principle, rather as a methodology that supports encapsulation and polymorphism.

Thus, abstraction is a fundamental idea, but it is not always regarded as a separate 'main' principle, as it is closely related to and implemented through other OOP principles.