“It was not possible for her to rejoin the unthinking world, becoming as she had been before, sleepily and smugly ignorant of consequences. There were thoughts that once thought, could not be unthought.”

Source: River of Blue Fire

Black God's Kiss (1934)
Context: Now she took the sword back into her hand and knelt on the rim of the invisible blackness below. She had gone this path once before and once only, and never thought to find any necessity in life strong enough to drive her down again. The way was the strangest she had ever known. There was, she thought, no such passage in all the world save here. It had not been built for human feet to travel. It had not been built for feet at all. It was a narrow, polished shaft that corkscrewed round and round. A snake might have slipped in it and gone shooting down, round and round in dizzy circles — but no snake on earth was big enough to fill that shaft. No human travelers had worn the sides of the spiral so smooth, and she did not care to speculate on what creatures had polished it so, through what ages of passage.

Source: Trusting Soul

Source: Way Station (1963), Ch. 16
Context: He had acted on an impulse, with no thought at all. The girl had asked protection and here she had protection, here nothing in the world ever could get at her. But she was a human being and no human being, other than himself, should have ever crossed the threshold.
But it was done and there was no way to change it. Once across the threshold, there was no way to change it.

Source: Gardens of the Moon (1999), Chapter 16 (p. 457)

Speech at the Nobel Banquet (1991)

At Last, Chapter 7

Part I, Ch. 17
To the Lighthouse (1927)