A self-evident axiom or premise; a pithy expression of a general principle or rule. (noun)
Examples of word maxim
This maxim is a wise guide to a great and simple precaution in life: Never, ever, think about something else when you should be thinking about the power of incentives …
 this maxim is as true as ever fell from poetical pen & there has more morality distilled from the waters of Helicon  than ever was procured from the withered skulls of metaphysicians or Philosophers.
And the old ‘first impressions’ maxim is one which cuts both ways, unfortunately for all concerned.
This is going to be awesome. anthony gadberry just read about this kid in maxim i think ... interesting story.
Yes, because they completely misunderstood what the beer-before-liquor maxim is about.
And this time, he likely has a better understanding than ever of the oft-quoted Mark Twain maxim: "Be good and you will be lonesome."
Note 47: The product of a progymnasmata exercise, gnome, a maxim, is an adaptation (abstraction or elaboration) of a preexisting moral statement.
Now, I do agree that a novel should be as long as a novel needs to be, but included within that maxim is the corollary that a novel should never be longer than it needs to be.
The flaw in Sagan's maxim is that an extraordinary claim is simply a claim about an extraordinary event and the occurrence of an extraordinary event does not necessarily entail that it would come with extraordinary evidence.