I keep not noticing earlier questions/answers - I do apologise, it doesn't indicate any contempt for fellow questioners/answerers on my part...

This summarises all that chalky answered before catching the train. I’ve taken out the wild guesses and repeated questions and added the clues at the end.

This is an * A B S T R A C T * thing.
It is (sort of) a human concept.
It can’t be heard.
Is it found in a book? Yes, but most things might, wouldn't you say?
It is common knowledge [but that's not the answer].
It is not commonly considered art.
It is not fictional.
It can be a saying or verbal expression.
Does it have to do with language? er ... No. It would be very misleading if I replied otherwise.
It is not directly something to do with the weather.
It is not an emotion.
It is not a feeling.
Does it have to do with music? – No.
Is it an action? - Not really ... it could be ... sometimes ... vague, sorry.
Nothing to do with water.
It is "not really" connected with the arts.
It would have been known about in 1900.
It is 'made' of abstract AND tangible things.
Is it a scientific theory or principle? - mmmm ... tricky. I shall say No, but your line of questioning might lead you to the right place.
It is not 'historically' attributed to one person.
It is not a religion.
It can exist/occur where there aren't any people around e.g. Mars.
Is it a rule or law? - Sort of, yes.
Is it anything to do with mathematics? - In a way, yes.
Is it possible for people to make one? - I think we've established that it's an 'it'.. therefore, it's a common perception that people can make 'it' - although they may not be aware they are.
It is not Gravity.
It is not Heat.
A quincunx? - What an interesting guess - it isn't correct but some might believe that the occurrence of such might have some bearing on 'it'.
Is it an effect rather than a cause? - It is both a cause and effect :-)
It can be perceived by any of the five senses.
It is not one of the seven deadly sins.
It is not a maze.
It is not the chaos theory.
Does it have something to do with the sun? - No, not really.
- Sorry - I've got a train to catch

[Clue #1] It is not an emotion but it can have an effect on the emotions.
[Clue#2] It is an 'it'.. therefore, it's a common perception that people can make 'it' - although they may not be aware they are.
“There are a whole bunch of questions that haven't been asked yet, which should simplify matters considerably”.

[Clue#3] - it has two sides - the one I'm thinking of is the happy/desirable side.