On the famous c2 wiki find "Alan Kay On Messaging". His words are from the year 1998 and still so true. And I am only close to the beginning of really understanding. But I have to, as usual, extract some of the key points for me. Make sure to read the entire message anyways, it's really very short!

Message header
Header of Alan Kay's message

I'm sorry that I long ago coined the term "objects" for this topic because it gets many people to focus on the lesser idea. The big idea is "messaging" - that is what the kernel of Smalltalk/Squeak is all about (and it's something that was never quite completed in our Xerox PARC phase).

This just means: make sure to understand how he means OO should be done. Need a starting point? Read poodr and listen to Sandi Metz's various talks findable online.

Inherently, once scratched on the surface of what OO means and reading about object calisthenics the question what is inheritance will stick out.
And Alan Key writes about it:

I would say that a system that allowed other meta things to be done in the ordinary course of programming (like changing what inheritance means, or what is an instance) is a bad design.

Two questions are raised here:

  1. What does inheritance mean?
  2. What is an instance?

/me has to go and read and answer those ... And work on improving what he calls to be

Given the current low state of programming in general

(which I understand as: most programming done nowadays is "bad" (that's how I interpret "low state") - maybe I am wrong)