Pho­to by NASA via Wiki­me­dia Com­mons

It is some­times said that sci­ence and phi­los­o­phy have grown so far apart that they no longer rec­og­nize each oth­er. Per­haps they no longer need each oth­er. And yet some of the most thought­ful sci­en­tists of modernity—those who most ded­i­cat­ed their lives not only to dis­cov­er­ing nature’s mys­ter­ies, but to com­mu­ni­cat­ing those dis­cov­er­ies with the rest of us—have been ful­ly steeped in a philo­soph­i­cal tra­di­tion. This espe­cial­ly goes for Carl Sagan, per­haps the great­est sci­ence com­mu­ni­ca­tor of the past cen­tu­ry or so.

Sagan wrote a num­ber of pop­u­lar books for lay­folk in which he indulged not only his ten­den­cies as a “hope­less roman­tic,” writes Maria Popo­va, but also as a “bril­liant philoso­pher.” He did not fear to ven­ture into the realms of spir­i­tu­al desire, and did not mock those who did like­wise; and yet Sagan also did not hes­i­tate to defend rea­son against “society’s most shame­less untruths and out­ra­geous pro­pa­gan­da.” These under­tak­ings best come togeth­er in Sagan’s The Demon-Haunt­ed World, a book in which he very patient­ly explains how and why to think sci­en­tif­i­cal­ly, against the very human com­pul­sion to do any­thing but.

In one chap­ter of his book, “The Fine Art of Baloney Detec­tion,” Sagan laid out his method, propos­ing what he called “A Baloney Detec­tion Kit,” a set of intel­lec­tu­al tools that sci­en­tists use to sep­a­rate wish­ful think­ing from gen­uine prob­a­bil­i­ty. Sagan presents the con­tents of his kit as “tools for skep­ti­cal think­ing,” which he defines as “the means to con­struct, and to under­stand, a rea­soned argu­ment and—especially important—to rec­og­nize a fal­la­cious or fraud­u­lent argu­ment.” You can see his list of all eight tools, slight­ly abridged, below. These are all in Sagan’s words:

• 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.

See the unabridged list at Brain Pick­ings, or read Sagan’s full chap­ter, ide­al­ly by get­ting a copy of The Demon-Haunt­ed World. As Popo­va notes, Sagan not only gives us suc­cinct instruc­tions for crit­i­cal think­ing, but he also makes a thor­ough list, with def­i­n­i­tions, of the ways rea­son fails us through “the most com­mon and per­ilous fal­lac­i­es of log­ic and rhetoric.” Sagan’s chap­ter on “Baloney Detec­tion” is, like the rest of the book, a high­ly lit­er­ary, per­son­al, engage­ment with the most press­ing sci­en­tif­ic con­sid­er­a­tions in our every­day life. And it is also an infor­mal yet rig­or­ous restate­ment of Aristotle’s clas­si­cal log­ic and rhetoric and Fran­cis Bacon’s nat­ur­al phi­los­o­phy.

Relat­ed Con­tent:

The Wis­dom of Carl Sagan Ani­mat­ed

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

Noam Chom­sky Calls Post­mod­ern Cri­tiques of Sci­ence Over-Inflat­ed “Poly­syl­lab­ic Tru­isms”

Josh Jones is a writer and musi­cian based in Durham, NC. Fol­low him at @jdmagness