The Key Concept of OOP is Indirection

I found an interesting introduction to OOP in an Objective-C book:
Mark Dalrymple, Scott Knaste. Learn Objective–C on the Mac.

An old saying in programming goes something like this, “There is no problem in computer science that can’t be solved by adding another level of indirection.” Indirection is a fancy word with a simple meaning—instead of using a value directly in your code, use a pointer to the value. Here’s a real-word example: you might not know the phone number of your favor- ite pizza place, but you know that you can look in the phone book to find it. Using the phone book like this is a form of indirection.

Indirection can also mean that you ask another person to do something rather than doing it yourself...

I've never thought about the programming in this way. But it is really so.