Mark Twain

Truth is the most valuable thing we have. Let us economize it. Samuel Langhorne Clemens, commonly known as Mark Twain, was an American author and satirist. He was also best buds with notorious inventor and scientist Nikola Tesla for a number of years; being an avid science enthusiast himself, Twain even participated in a number of experiments with Tesla.

Writings
Twain had multiple careers throughout his life, but he had his greatest success as a prolific writer. He first gained notice with his humorous story "The Celebrated Jumping Frog of Calaveras County" in 1865. His later books, The Adventures of Tom Sawyer (1876) and Adventures of Huckleberry Finn (1884) brought him fame as well as fortune. Huckleberry Finn was first banned in 1885 by the Concord, Massachusetts library. The early bannings were mostly because of what was called, "backwards talking", that is writing dialogue as people actually spoke rather than in refined English. Bannings and attempted bannings of the book have continued apace to the present day, but the rationale changed at some point. Most objections to the book in modern times, including by the NAACP in the 1950s, are because it uses the word 'nigger' frequently (213 times) and for its alleged racism. The book itself is set in the antebellum South, and is actually about escaping slavery, escaping an abusive father, racism, and the meaning of freedom.

Views
Through his writings, Twain appears to have been a Presbyterian, a deist, and an atheist, as his views changed over time.

Christian Science
Mark Twain was a staunch opponent of Christian Science. He wrote an essay called (appropriately enough) Christian Science, which criticized the religion and explains its "miracles" as the results of the placebo effect (many of the essay's points are also relevant to much of modern alternative medicine). As can be judged by the essay and Twain's biographers, Twain apparently did have some belief that mental healing could work but disclaimed Christian Science over its charging money, which he logically pointed out conflicts with their doctrine, as humorously shown from this dialogue the narrator has with a Christian Scientist woman who "treated" him:

""Nothing exists but Mind?"

"Nothing," she answered. "All else is substanceless, all else is imaginary."

I gave her an imaginary check, and now she is suing me for substantial dollars. It looks inconsistent."

- Christian Science

He also objected to the way Christian Science forbade its users from using actual medicine and "claim[ed] ability to cure every conceivable human ailment through the application of [its] mental forces alone." Being well aware that the placebo effect could only work for "imagination-manufactured disease" (that is, subjective symptoms, but not physical causes), Twain noted: "There would seem to be an element of danger here."

Mormonism
Passing through Utah in his trip to the American West, Twain came in contact with a number of Mormons during a fairly early stage of their history. He found the religion mostly harmless, if somewhat odd.

Creationism
Twain's Extracts from Adam's and Eve's Diaries humorously recounted the adventures of the first people in the Garden of Eden/Niagara Falls Park. Adam notes of Eve, "Nothing ever satisfies her but demonstration; untested theories are not in her line, and she won't have them." Adam mentions that Eve thinks the buzzard "was intended to live on decayed flesh" and that she vows "to study out why the animals called lions and tigers live on grass and flowers, when, as she says, the sort of teeth they wear would indicate that they were intended to eat each other."

In a 1903 article titled "Was the World Made for Man?" Twain argues that it was not.

ESP
Twain was a believer in telepathy. In his essay Mental Telegraphy Again, Twain recounted a few experiences of his which he believed supported the existence of ESP.

Homeopathy
Some homeopathists claim that Twain was a supporter of homeopathy, and cite the quote: "The introduction of homeopathy forced the old school doctor to stir around and learn something of a rational nature about his business. You may honestly feel grateful that homeopathy survived the attempts of allopathists to destroy it."

The quote is from his essay A Majestic Literary Fossil, which is about old-fashioned forms of quackery, and this is it in context:

In other words, he says that, thankfully, homeopathy pushed medicine to evolve beyond the remedies of traditional medicine – which is not quite the same thing as saying that homeopathy itself is any good.

The essay also includes this interesting passage about the recent progress of science (and medicine in particular):

Anti-imperialism
Twain strongly opposed US involvement with the Spanish-American War, and the Philippine-American War which followed when the Filipinos rebelled against the US taking possession of their country following its "liberation." He was a member of the American Anti-Imperialist League. Some of his anti-war writings remained unpublished until after his death due to controversy. An example of his criticism of American imperialism regarding the Philippine-American War can be seen below, in which he likens it to a "robbing expedition":

He wrote a short story at the time as well called "The War Prayer" which harshly criticized Christians who supported wars in a way Twain thought to be blatantly contradiction of their own supposed principles (such as the Golden Rule). In this, after a sermon where the minister praises a war going on at the time and calls on young men to participate, an angel in the form of an old man enters the church. He tells them God is going to answer their prayer, but they don't really understand its whole import. The angel says that when they pray for victory, they're praying for injuries and deaths to enemy soldiers, along with the devastation of their loved ones. If they really still want that, the angel says to pray on. They dismiss him once he leaves, thinking that he's just a crazy man.

Anti-Stratfordianism
Like some public intellectuals of the 19th century and later eras, Mark Twain believed in the widely-discredited hogwash that William Shakespeare of Stratford upon Avon did not in fact write any of the works attributed to him. This is of course utter and complete rubbish, but sadly many cranks who believe similar nonsense like to quote him. Now there is only one question: How could such a half literate Southerner like Samuel Langhorne Clemens whose business ventures all ended in disaster possibly have written the elaborate works of Mark Twain with their exotic locales, their sympathetic portrayal of black people and Native Americans and several things Samuel Langhorne Clemens cannot possibly have known about?

Criticism of religion
Late in his life, Twain became increasingly critical of belief in gods, Christianity and religion generally. He wrote, for instance:

This line was from his book The Mysterious Stranger—admittedly it's said by Satan (who is a character in the story). In another book, he writes about a man who attempts to be "like God", inflicting all manner of injuries, diseases and misfortunes on people. He deemed faith "believing what you know ain't so", in general disdaining religion as a negative force.