Write one true sentence, counselled Hemingway. I didn't get the deepest truth out of the idea, complained Mansfield. In fiction writing, the main thing is truth. That is what we, as writers, struggle to convey.
What is "truth" in fiction? To me it is reproducing a mood, character or storyline that the reader recognizes. Bonus points if you can articulate a feeling the reader has experienced. This is truth: that moment of recognition. When the reader thinks "yes!"
Claire Messud fleetingly tackles this question during this interview for the Harvard Gazette. She identifies "these moments that have that energy, you recognize is as a reader... if you recognize a work of literature as true, it has an energy and an authority. And as readers, we want more of that. You will read 1,000 pages or you will stay up until 4 a.m. to have that."