Not in sweet sins but in a dreary flickering of the mind over it knows not what and knows not why

From The Screwtape Letters by C.S. Lewis, h/t Ann Althouse. Letter XII.

Recalling that the context is one of the lesser devil's writing to his nephew, Screwtape, offering advice on how to lead people astray and away from the Enemy (God).

As this condition becomes more fully established, you will be gradually freed from the tiresome business of providing Pleasures as temptations. As the uneasiness and his reluctance to face it cut him off more and more from all real happiness, and as habit renders the pleasures of vanity and excitement and flippancy at once less pleasant and harder to forgo (for that is what habit fortunately does to a pleasure) you will find that anything or nothing is sufficient to attract his wandering attention. You no longer need a good book, which he really likes, to keep him from his prayers or his work or his sleep; a column of advertisements in yesterday's paper will do. You can make him waste his time not only in conversation he enjoys with people whom he likes, but in conversations with those he cares nothing about on subjects that bore him. You can make him do nothing at all for long periods. You can keep him up late at night, not roistering, but staring at a dead fire in a cold room. All the healthy and outgoing activities which we want him to avoid can be inhibited and nothing given in return, so that at last he may say, as one of my own patients said on his arrival down here, "I now see that I spent most of my life in doing neither what I ought nor what I liked". The Christians describe the Enemy as one "without whom Nothing is strong". And Nothing is very strong: strong enough to steal away a man's best years not in sweet sins but in a dreary flickering of the mind over it knows not what and knows not why, in the gratification of curiosities so feeble that the man is only half aware of them, in drumming of fingers and kicking of heels, in whistling tunes that he does not like, or in the long, dim labyrinth of reveries that have not even lust or ambition to give them a relish, but which, once chance association has started them, the creature is too weak and fuddled to shake off.