Though he died too young, Carl Sagan left behind an impres­sive­ly large body of work, includ­ing more than 600 sci­en­tif­ic papers and more than 20 books. Of those books, none is more wide­ly known to the pub­lic — or, still, more wide­ly read by the pub­lic — than Cos­mos, accom­pa­nied as it was by Cos­mos: A Per­son­al Voy­age, a com­pan­ion tele­vi­sion series on PBS. Sagan’s oth­er pop­u­lar books, like Shad­ows of For­got­ten Ances­tors or Con­tact (the basis of the 1997 Hol­ly­wood movie) are also well worth read­ing, but we per­haps ignore at our great­est per­il The Demon-Haunt­ed World: Sci­ence as a Can­dle in the Dark. Pub­lished in 1995, the year before Sagan’s death, it stands as his tes­ta­ment to the impor­tance of crit­i­cal, sci­en­tif­ic think­ing for all of us.

*The Demon-Haunt­ed World*is the sub­ject of the Genet­i­cal­ly Mod­i­fied Skep­tic video above, whose host Drew McCoy describes it as his favorite book. He pays spe­cial atten­tion to its chap­ter in which Sagan lays out what he calls his “baloney detec­tion kit.” This assem­bled metaphor­i­cal box of tools for diag­nos­ing fraud­u­lent argu­ments and con­struct­ing rea­soned ones involves these nine prin­ci­ples:

• Wher­ev­er pos­si­ble there must be inde­pen­dent con­fir­ma­tion of the “facts.”
• Encour­age sub­stan­tive debate on the evi­dence by knowl­edge­able pro­po­nents of all points of view.
• 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.
• 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.
• 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.
• 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.
• If there’s a chain of argu­ment, every link in the chain must work (includ­ing the premise) — not just most of them.
• 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. As McCoy points out, these tech­niques of mind have to do with can­cel­ing out the man­i­fold bias­es present in our think­ing, those nat­ur­al human ten­den­cies that incline us to accept ideas that may or may not coin­cide with real­i­ty as it is. If we take no trou­ble to cor­rect for these bias­es, Sagan came to believe, we’ll become easy marks for all the trick­sters and char­la­tans who hap­pen to come our way. And that’s just on the micro lev­el: on the macro lev­el, vul­ner­a­bil­i­ty to delu­sion can bring down entire civ­i­liza­tions.

“Like all tools, the baloney detec­tion kit can be mis­used, applied out of con­text, or even employed as a rote alter­na­tive to think­ing,” Sagan cau­tions. “But applied judi­cious­ly, it can make all the dif­fer­ence in the world — not least in eval­u­at­ing our own argu­ments before we present them to oth­ers.” McCoy urges us to heed these words, adding that “this kit is not some per­fect solu­tion to the world’s prob­lems, but as it’s been uti­lized over the last few cen­turies” — for its basic pre­cepts long pre­date Sagan’s par­tic­u­lar artic­u­la­tion — “it has enabled us to cre­ate tech­no­log­i­cal inno­va­tions and use­ful explana­to­ry mod­els of our world more quick­ly and effec­tive­ly than ever before.” The walls of baloney may always be clos­ing in on human­i­ty, but if you fol­low Sagan’s advice, you can at least give your­self some breath­ing room.

Relat­ed con­tent:

Carl Sagan on the Impor­tance of Choos­ing Wise­ly What You Read (Even If You Read a Book a Week)

Carl Sagan’s Syl­labus & Final Exam for His Course on Crit­i­cal Think­ing (Cor­nell, 1986)

Carl Sagan Pre­dicts the Decline of Amer­i­ca: Unable to Know “What’s True,” We Will Slide, “With­out Notic­ing, Back into Super­sti­tion & Dark­ness” (1995)

Richard Feyn­man Cre­ates a Sim­ple Method for Telling Sci­ence From Pseu­do­science (1966)

How to Spot Bull­shit: A Man­u­al by Prince­ton Philoso­pher Har­ry Frank­furt (RIP)

Crit­i­cal Think­ing: A Free Course

Based in Seoul,Col­inMarshall writes and broad­cas**ts on cities, lan­guage, and cul­ture. His projects include the Sub­stack newslet­ter Books on Cities* and the book*The State­less City: a Walk through 21st-Cen­tu­ry Los Ange­les. Fol­low him on the social net­work for­mer­ly known as Twit­ter at @colinmarshall.