Mine was a trained Presbyterian conscience and knew but the one duty--to hunt and harry its slave upon all pretexts and on all occasions, particularly when there was no sense nor reason in it.
- Autobiography of Mark Twain

An uneasy conscience is a hair in the mouth.
- Notebook, 1904

It takes up more room than all the rest of a person's insides, and yet ain't no good nohow.
- Adventures of Huckleberry Finn

If you grant that one man's conscience doesn't know right from wrong, it is an admission that there are others like it. This single admissions pulls down the whole doctrine of infallibility of judgment in consciences.
- What is Man?

In those old slave-holding days the whole community was agreed as to one thing--the awful sacredness of slave property. To help steal a horse or a cow was a low crime, but to help a hunted slave, or feed him or shelter him, or hide him, or comfort him, in his troubles, his terrors, his despair, or hesitate to promptly to betray him to the slave-catcher when opportunity offered was a much baser crime, & carried with it a stain, a moral smirch which nothing could wipe away. That this sentiment should exist among slave-owners is comprehensible--there were good commercial reasons for it--but that it should exist & did exist among the paupers, the loafers the tag-rag & bobtail of the community, & in a passionate & uncompromising form, is not in our remote day realizable. It seemed natural enough to me then; natural enough that Huck & his father the worthless loafer should feel it & approve it, though it seems now absurd. It shows that that strange thing, the conscience--the unerring monitor--can be trained to approve any wild thing you want it to approve if you begin its education early & stick to it.
- Notebook #35 (reprinted in Adventures of Huckleberry Finn, Univ. of California Press, 2003)

If I had the remaking of man, he wouldn't have any conscience. It is one of the most disagreeable things connected with a person; and although it certainly does a great deal of good, it cannot be said to pay, in the long run; it would be much better to have less good and more comfort. Still, this is only my opinion, and I am only one man; others, with less experience, may think differently. They have a right to their view. I only stand to this: I have noticed my conscience for many years, and I know it is more trouble and bother to me than anything else I started with. I suppose that in the beginning I prized it, because we prize anything that is ours; and yet how foolish it was to think so. If we look at it in another way, we see how absurd it is: if I had an anvil in me would I prize it? Of course not. And yet when you come to think, there is no real difference between a conscience and an anvil--I mean for comfort. I have noticed it a thousand times. And you could dissolve an anvil with acids, when you couldn't stand it any longer; but there isn't any way that you can work off a conscience--at least so it will stay worked off; not that I know of, anyway.
- A Connecticut Yankee in King Arthur's Court

All the consciences I have ever heard of were nagging, badgering, fault-finding, execrable savages! Yes; and always in a sweat about some poor little insigificant trifle or other--destruction catch the lot of them, I say! I would trade mine for the small-pox and seven kinds of consumption, and be glad of the chance.
- "The Facts Concerning the Recent Carnival of Crime in Connecticut"

It seems to me that a man should secure the well done, faithful servant, of his own conscience first and foremost, and let all other loyalties go.
- Consistency speech, 1887

Good friends, good books and a sleepy conscience: this is the ideal life.
- Mark Twain's Notebook; also in More Maxims of Mark, Merle Johnson, 1927

Conscience, man's moral medicine chest.
- Mark Twain's Autobiography