The key ideas

The Demon-Haunted World: Science as a Candle in the Dark

Carl Sagan, 1995

Why do smart, curious people believe in alien abductions, the face on Mars and healing crystals? Astronomer Carl Sagan takes these stories apart one by one and shows where our eyes, our memory and our wish to believe let us down. He offers a simple toolkit against nonsense and explains why the ability to doubt matters not only to scientists but to a free society.

Who it is for

For anyone who wants to tell real science from the fake kind and stop falling for flashy sensations. And for anyone curious why smart people believe in aliens, witches and prophetic dreams.

Now, what's the difference between an invisible, incorporeal, floating dragon who spits heatless fire and no dragon at all?

The driver who wanted to know everything about science

Carl Sagan flew in for a conference where scientists and television people were discussing how best to show science on TV. A driver met him at the airport. When he found out who his passenger was, he lit up: he had so many questions about science! His name, Sagan writes, was William Buckley, just like the famous television host.

All through the long drive he questioned Sagan. But not about science. He wanted to know about frozen aliens kept at an air force base, about talking to the spirits of the dead, about healing crystals, the prophecies of Nostradamus, astrology and lost Atlantis. Time after time Sagan answered that the evidence did not hold up. And he knew almost nothing about real science: nothing about DNA, nothing about what quantum mechanics is.

Sagan does not think he was stupid. He was a smart, curious man. Real science had simply been filtered out on its way to him. School, television and the newspapers had handed him a fake and never taught him how to tell it from the real thing. That is what The Demon-Haunted World, published in 1995, is about: learning to separate what you want to believe from what stands up to testing.

Ahead are the face on Mars, an invisible dragon in a garage, tales of alien abduction, witch hunts, a toolkit against nonsense and a boy who was forbidden to learn to read. Hence the book's subtitle: science as a candle in the dark.

Facts can be memorized, but thinking has to be learned

The book's central idea sounds simple: science is first of all a way of thinking, and only then a collection of facts. Facts can be memorized, like the multiplication table. A way of thinking works differently. It asks you to test every idea, even the most beautiful one, and to let it go if it fails the test.

That is why, in Sagan's view, science has an error-correcting machinery built into it. No authority can simply declare something true. Great scientists made mistakes too, and those mistakes were found by others, by people who took the trouble to check again.

This leads to a strange feature of science that many people find hard to accept. It almost never says "for certain". Sagan reminds us that absolute certainty exists only in pure mathematics. Everything else is knowledge with caveats, and it gets sharper with every new test.

For an ordinary person this matters a great deal. Someone who has learned only the conclusions of science is defenseless against a fake: they have no way to tell one loud claim from another. Someone who understands how ideas get tested can make sense even of things they have never studied.

Sagan adds a worrying argument. Transport, communications, medicine, agriculture and even elections now depend on science and technology. Yet very few people understand them. He calls this mix of ignorance and power a prescription for disaster.

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About the author

Carl Sagan was an American astronomer and science communicator, a professor at Cornell University and took part in NASA's planetary exploration programs. He created the television series Cosmos and won the Pulitzer Prize for his book The Dragons of Eden.

All books by this author: Carl Sagan

Terms from the book

  • Kramer
  • Occam's razor
  • hypnosis
  • medium
  • Baltimore
  • DNA
  • NASA
  • Quantum mechanics
  • Viking
  • Alvarez
  • Skepticism
  • Buckley: The driver who took Sagan to a conference. He had the same name as the well-known American writer and television host William F. Buckley.
  • Nostradamus: Michel de Nostredame, a sixteenth-century French physician and astrologer. People still try to link his vague rhyming "prophecies" to real events.
  • Astrology: The belief that the positions of the stars and planets shape a person's fate and character. There is no scientific evidence for it.
  • Atlantis: A legendary sunken continent first described by Plato. Geology tells us no such continent ever existed.
  • Cydonia: A region in the northern hemisphere of Mars where people spotted a "face" in images taken in 1976.
  • Pareidolia: The tendency to see familiar shapes, most often faces, in random patterns: clouds, stains, shadows.
  • Roper: An American company that runs public opinion polls.
  • Sleep paralysis: A state on the border between sleep and waking, when a person is already conscious but cannot move. It often comes with fear and vivid visions.
  • Incubus: In medieval belief, incubi and succubi were demons who came to sleepers at night.
  • Inquisitor: A church judge who investigated cases of heresy and witchcraft.
  • Sprenger: Jakob Sprenger, a fifteenth-century German Dominican friar and theologian. He is traditionally named as co-author of the Malleus Maleficarum.
  • Malleus Maleficarum: The Hammer of Witches, a late fifteenth-century treatise on how to identify and punish witches. It became the handbook of witch hunters across Europe.
  • Microbiology: The science of microorganisms: bacteria, viruses, fungi. Among other things, it explains what causes many diseases.
  • Meteorology: The science of the atmosphere and the weather.
  • Ady: Thomas Ady, a seventeenth-century English writer. In his book A Candle in the Dark (1656) he spoke out against the witch hunts.
  • Druyan: Ann Druyan, an American writer and producer, Carl Sagan's wife. She co-wrote several chapters of this book with him.
  • Felberbaum: Arthur Felberbaum, a friend of Sagan and Druyan. According to Druyan, the phrase "baloney detection kit" was his.
  • Bower: Doug Bower, an Englishman who secretly made crop circles for years with Dave Chorley and confessed to the hoax in 1991.
  • Chorley: Dave Chorley, Doug Bower's partner in the crop circle hoax.
  • Randi: James Randi, a Canadian-born American magician and famous skeptic. For decades he exposed psychics and mediums.
  • Auld: Hugh Auld, young Frederick's master in Baltimore. His wife Sophia began teaching the boy to read.
  • Douglass: Frederick Douglass, an American orator and writer who escaped from slavery. One of the leading figures in the fight to abolish slavery in the United States.
  • Bill of Rights: The first ten amendments to the United States Constitution. They protect freedom of speech, religion and the press, along with other rights of citizens.

All terms

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