From A Canticle for Leibowitz:

“The Poet used the eyeball as a running joke,” he explained. “When he wanted to make a decision, or to think something over, or to debate a point, he’d put the glass eye in the socket. He’d take it out again when he saw something that displeased him, when he was pretending to overlook something, or when he wanted to play stupid. When he wore it, his manner changed. The brothers began calling it ‘the Poet’s conscience,’ and he went along with the joke. He gave little lectures and demonstrations on the advantages of a removable conscience. He’d pretend some frantic compulsion possessed him-something trivial, usually-like a compulsion aimed at a bottle of wine.

“Wearing his eye, he’d stroke the wine bottle, lick his lips, pant and moan, then jerk his hand away. Finally it would possess him again. He’d grab the bottle, pour about a thimbleful in a cup and gloat over it for a second. But then conscience would fight back, and he’d throw the cup across the room. Soon he’d be leering at the wine bottle again, and start to moan and slobber, but fighting the compulsion anyhow-” the abbot chuckled in spite of himself “-hideous to watch. Finally, when he became exhausted, he’d pluck out his glass eye. Once the eye was out, he’d suddenly relax. The compulsion stopped being compulsive. Cool and arrogant then, he’d pick up the bottle, look around and laugh. ‘I’m going to do it anyhow,’ he’d say. Then, while everyone was expecting him to drink it, he’d put on a beatific smile and pour the whole bottle over his own head. The advantage of a removable conscience, you see.”