Going from A something to a THE something:
A general truth can be a very simple idea but difficult to follow.
For example, a watched pot never boils. A pot with water in it you are watching as it boils.
The pot [the one I mentioned above as a general proposition] does not boil is just a perception; the water or liquid in it will, in fact, boil at some point but will seem like a long time if you are watching it instead of doing something else while it getting to the boiling poin.
No article is used with about abstract nouns; an article would be used only if the abstract notion is in the form X of Y. The beauty of this principle is that it is simple.
Truth is difficult to understand even though philosophers have discussed it through the ages.
Beauty is a hallmark of Renaissance painting.
Rudeness on the telephone can come back to haunt you.
They achieved great **insight into the problem**.
You can add adjectives to the abstract nouns but they still do not take a or the.
The public statues in Ancient Greece were known for their great beauty.
But here we have an exception: truth can be an abstract uncountable noun or it can be countable.
For example: "Truth is difficult to understand" versus A truth about him is that he is very clever, in fact. [general] Also: In life, general truths [countable] are hard to accept. BUT: The truth of the matter is that I don't like strawberries. [x of y, abstract noun].
The fact that you're reading this is a general truth. [countable noun, general statement]
Can there be empirical evidence for general truth? [abstract noun, no article].
The general truth of the experience was that I hated it. [abstract noun in the X of Y form].
Finally, a truth can be the truth if it is specific to a situation:
The truth is one has to work really hard to sort out the use of articles versus no articles in English.
The idea here is: there is a specific truth to the situation regarding use of articles versus no articles in English.