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A Brief Manual of Skepticism, Courtesy of Carl Sagan


Whether or not you’re dedicated to science, these tips to identify fallacies apply to any form of rigorous thinking.

Carl Sagan was one of the greatest popularizers of science to have ever existed. Today, though, the word “popularizer” is undervalued. It sounds like something “cheap,” “vulgar” and yes, even “popular.” It’s like something distributed among the common people, made available to all people regardless of their condition, because the stars are there for all of us to see, as we’re all together aboard this small blue dot we still call Earth.

In a nobler sense, a “popularizer” is someone who distributes knowledge among the people. It’s a matter of generosity, and in the case of science, a matter of cultivating critical thinking.

In one of his most popular books, The Demon-Haunted World, Sagan shared a “Baloney detection kit,” a kind of brief for identifying the propaganda and half-truths that populate the media for years. Here we offer a condensed version, though the entire chapter is available here.

  1. Wherever possible there must be independent confirmation of the “facts.”
  2. Encourage substantive debate on the evidence by knowledgeable proponents of all points of view.
  3. Arguments from authority carry little weight — “authorities” have made mistakes in the past. They will do so again in the future. Perhaps a better way to say it is that in science there are no authorities; at most, there are experts.
  4. Spin more than one hypothesis. If there’s something to be explained, think of all the different ways in which it could be explained. Then think of tests by which you might systematically disprove each of the alternatives.
  5. Try not to get overly attached to a hypothesis just because it’s yours. It’s only a way station in the pursuit of knowledge. Ask yourself why you like the idea. Compare it fairly with the alternatives. See if you can find reasons for rejecting it. If you don’t, others will.
  6. If whatever it is you’re explaining has some measure, some numerical quantity attached to it, you’ll be much better able to discriminate among competing hypotheses. What is vague and qualitative is open to many explanations.
  7. If there’s a chain of argument, every link in the chain must work (including the premise) — not just most of them.
  8. Occam’s Razor. This convenient rule-of-thumb urges us when faced with two hypotheses that explain the data equally well to choose the simpler. Always ask whether the hypothesis can be, at least in principle, falsified…. You must be able to check assertions out. Inveterate skeptics must be given the chance to follow your reasoning, to duplicate your experiments and see if they get the same result.

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