The problem with "essences" of things is that nothing has an essence out of context. There's no absolute, unchanging essence. The "essence" changes by context. For each context, something things are important and others aren't. An "essence" isn't supposed to change depending what you're interested in today.
For whatever problem you want to solve, some things will be important and others won't be. There's no one essence of what's important every time, for all problems, in all contexts.
There is something kinda similar which is a good concept. Some aspects of a thing are important for many problems, for many contexts. They have reach. They are non-parochail, non-accidental. They keep coming up.
This concept is *not* what essence means:
> essence |ˈesəns|
> noun
> the intrinsic nature or indispensable quality of something, esp. something abstract, that determines its character: conflict is the essence of drama.
> • Philosophy a property or group of properties of something without which it would not exist or be what it is.
So what should we call it? "Essentials" or "fundamentals" is not at all perfect but not too bad. Much better than essences. I'm not sure if any ideal word is going to exist -- with a lot of good epistemology you will find all the English words have some mistakes in them.
> • (essentials) the fundamental elements or characteristics of something:he was quick to grasp the essentials of an opponent's argument.
It's OK to use words that aren't perfect as long as the main point of the word is the right concept and the mistake is more of a superficial taint than being integral to the word.
Essences don't pass this test. Essentials does acceptably well. People say "essentials" to mean "the important parts" or "the parts suitable to be principles" or whatever. That's the main idea and it's OK.
That essential can also mean "absolutely necessary" is a mistake but I don't think it ruins the word. I don't think everyone takes the word to be absolute or wouldn't know how the concept could work without absolutes.
> 1 absolutely necessary; extremely important: [ with infinitive ] : it is essential to keep up-to-date records | fiber is an essential ingredient.
With essences, though, the main idea is wrong, it's much harder to fix the word.
-- Elliot Temple
http://elliottemple.com/