The Uncle Charles principle is a particular occurrence in Point of View when the POV effectively slips out of omniscience into the particular, subjective view of a character.
It was so named by the American critic Hugh Kenner after Uncle Charles in Portrait of the Artist as a Young Man by James Joyce. In the opening of part two of the book, Joyce writes:
"Every morning, therefore, uncle Charles repaired to his outhouse...."
Another critic objected to this as poor language, citing the word "repair" as archaic and pompous. Exactly, said Joyce. This is exactly the sort of word that Uncle Charles would use, and it is therefore entirely appropriate. In other words, it is as though the narration is now coming through Uncle Charles.
Another example can be found in Miss Brill by Katherine Mansfield:
Miss Brill was glad that she had decided on her fur. The air was motionless, but when you opened your mouth there was just a faint chill, like a chill from a glass of iced water before you sip, and now and again a leaf came drifting–from nowhere, from the sky. Miss Brill put up her hand and touched her fur. Dear little thing! It was nice to feel it again.
That "Dear Little Thing!" is clearly Miss Brill speaking, but it isn't in quotes. For the moment the narrative has slipped entirely into her point of view.
Cormac McCarthy, of course, takes this to extremes, as he does with most things, particularly in Suttree which is full of instances when the third person narrative not only adopts the Uncle Charles principle but actually steps entirely into the first person, and the omniscient narrator and the main protagonist, Suttree, become one. Yet another twin for that most reluctant of twins, Buddy Suttree. In this example, Suttree is shown some old photographs of long lost kin and the experience is so traumatic we are drawn into his own, horrified thoughts, so that by the end of the passage we are in first person:
She came with the tea, a tall vase full, chocked with ice, a curl of lemon. He ladled sugar in. Between the mad hag’s face and this young girl a vague stellar drift, the wheeling of planets on their ether trunnions. Likeness of lost souls haunt us from old chromos and tintypes brown with age. Bloodless skull and dry white hair, matriarchal meat drawn lean and dry on frail bone, a bitter refund ashen among silk and lilies by candlelight in a cold hall, black lacquered bier on sawhorses wound with crepe. I would not cry. My sisters cried.
It's a hard technique to get right. Suttree is a masterpiece of writing.