“I believe that the mind can be permanently profaned by the habit of attending to trivial things, so that all our thoughts shall be tinged with triviality.”
Life Without Principle (1863)
Context: It is so hard to forget what it is worse than useless to remember! If I am to be a thoroughfare, I prefer that it be of the mountain-brooks, the Parnassian streams, and not the town-sewers. There is inspiration, that gossip which comes to the ear of the attentive mind from the courts of heaven. There is the profane and stale revelation of the bar-room and the police court. The same ear is fitted to receive both communications. Only the character of the hearer determines to which it shall be open, and to which closed. I believe that the mind can be permanently profaned by the habit of attending to trivial things, so that all our thoughts shall be tinged with triviality.
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Henry David Thoreau 385
1817-1862 American poet, essayist, naturalist, and abolitio… 1817–1862Related quotes

Source: The Divine Conspiracy: Rediscovering Our Hidden Life In God

Source: Metaphysics: Concept and Problems

"What I Believe" in The Forum 84 (September 1930), p. 139; some of these expressions were also used separately in other Mencken essays.
1930s
Context: I believe that religion, generally speaking, has been a curse to mankind — that its modest and greatly overestimated services on the ethical side have been more than overcome by the damage it has done to clear and honest thinking.
I believe that no discovery of fact, however trivial, can be wholly useless to the race, and that no trumpeting of falsehood, however virtuous in intent, can be anything but vicious.
I believe that all government is evil, in that all government must necessarily make war upon liberty and the democratic form is as bad as any of the other forms.
I believe that the evidence for immortality is no better than the evidence of witches, and deserves no more respect.
I believe in the complete freedom of thought and speech — alike for the humblest man and the mightiest, and in the utmost freedom of conduct that is consistent with living in organized society.
I believe in the capacity of man to conquer his world, and to find out what it is made of, and how it is run.
I believe in the reality of progress.
I —But the whole thing, after all, may be put very simply. I believe that it is better to tell the truth than to lie. I believe that it is better to be free than to be a slave. And I believe that it is better to know than be ignorant.
Preface (1982), p. xv.
Europe and the People Without History, 1982

“People are curious. A few people are. They will be driven to find things out, even trivial things.”
"Friend of My Youth", in Friend of My Youth https://books.google.com/books?id=JHO0R0im-WsC&pg=PT94 (1990)
Context: People are curious. A few people are. They will be driven to find things out, even trivial things. They will put things together, knowing all along that they may be mistaken. You see them going around with notebooks, scraping the dirt off gravestones, reading microfilm, just in the hope of seeing this trickle in time, making a connection, rescuing one thing from the rubbish.
And they may get it wrong, after all. I may have got it wrong.