Book review: The Sleepwalkers by Arthur Koestler

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The progress of Science is gen­er­ally re­garded as a kind of clean, ra­tio­nal ad­vance along a straight as­cend­ing line; in fact it has fol­lowed a zigzag course, at times al­most more be­wil­der­ing than the evolu­tion of poli­ti­cal thought. The his­tory of cos­mic the­o­ries, in par­tic­u­lar, may with­out ex­ag­ger­a­tion be called a his­tory of col­lec­tive ob­ses­sions and con­trol­led schizophre­nias; and the man­ner in which some of the most fun­da­men­tal dis­cov­er­ies were ar­rived at re­minds one more of a sleep­walker’s perfor­mance than an elec­tronic brain’s.
Arthur Koestler

The Sleep­walk­ers is an en­light­en­ing his­tory of as­tron­omy from the An­cient Greeks to New­ton. It par­tic­u­larly fo­cuses on three char­ac­ters who shifted sci­en­tific con­sen­sus from the Ptole­maic geo­cen­tric model of the so­lar sys­tem to a he­lio­cen­tric one: Coper­ni­cus, Ke­pler, and Gal­ileo. And char­ac­ters is the right word, be­cause Koestler digs into their per­sonal quirks and foibles with gusto. If he is to be be­lieved, these three key sci­en­tists were all tem­per­a­men­tal to the point of self-de­struc­tive­ness.

Coper­ni­cus and Kepler

Coper­ni­cus, ac­cord­ing to Koestler, was “a man of the Mid­dle Ages: haunted by its anx­ieties, rid­den with its com­plexes, a timid, con­ser­va­tive cleric, who started the rev­olu­tion against his will.” The ori­gin of his he­lio­cen­tric sys­tem was in fact his at­tempt to fix an aes­thetic flaw which made Ptolemy’s sys­tem “nei­ther suffi­ciently ab­solute nor suffi­ciently pleas­ing”: that it mod­el­led planets as mov­ing at vari­able speeds. In do­ing so, he was in­spired by An­cient Greek writ­ings, and in par­tic­u­lar refer­ences to the he­lio­cen­tric sys­tem that Her­a­clides of Pon­tos and Aristarchus of Samos pro­posed in the 3rd cen­tury BC. Coper­ni­cus was ter­rified by the prospect of crit­i­cism, so that it took decades for him to pub­lish his con­clu­sions (On the rev­olu­tions of the ce­les­tial spheres only came out just be­fore his death in 1543). But in fact, the pub­li­ca­tion it­self was fairly ir­rele­vant: it was te­dious and ac­tu­ally more com­pli­cated than ex­ist­ing geo­cen­tric sys­tems. As pre­vi­ous as­tronomers had done, Coper­ni­cus ex­plained com­plex-look­ing or­bits us­ing mod­els in which planets ro­tated on cir­cles which them­selves ro­tated in cir­cles (known as epicy­cles). Epicy­cles were a very pow­er­ful mod­el­ling tool (in fact, equiv­a­lent to Fourier se­ries!) but made the­o­ries much uglier and more com­plex—and Coper­ni­cus needed more of them than pre­vi­ous geo­cen­tric mod­els. But the im­por­tant thing was that he­lio­cen­trism, an idea which been float­ing around with­out much fan­fare since the Greeks, was brought to greater promi­nence—in­spiring oth­ers, par­tic­u­larly Ke­pler and Gal­ileo.

Ke­pler was prone to bursts of pas­sion, al­ter­nat­ing be­tween fury and self-de­p­re­ca­tion. He was also driven pri­mar­ily by a philo­soph­i­cal taste verg­ing on mys­ti­cism: his first sys­tem of as­tron­omy matched the six known planets with the five perfect Pla­tonic solids for aes­thetic rea­sons, and was com­plete bunk. So too was his the­ory that the po­si­tions of the planets were de­ter­mined by the ra­tios of mu­si­cal har­monies. Yet these in­stincts kept him push­ing, mostly by ac­ci­dent, in the right di­rec­tion. His break­through came when given ac­cess to the metic­u­lous ob­ser­va­tions of Ty­cho Brahe (also quite a char­ac­ter: af­ter his nose was chopped off in a duel, he wore a silver re­place­ment). The two had a stormy re­la­tion­ship, but af­ter Ty­cho’s death Ke­pler slogged through years of calcu­la­tions to try and pin down the shape of plane­tary or­bits. Through de­ter­mi­na­tion and luck (he made sev­eral er­rors which for­tu­nately can­cel­led out), he even­tu­ally for­mu­lated his three laws: that planets move in el­lipses, with the sun at one fo­cal point; that they travel at vari­able speeds such that the line be­tween them and the sun sweeps out the same area per unit time; and that the squares of the pe­ri­ods of the planets’ rev­olu­tions are pro­por­tional to the cubes of their mean dis­tances from the sun.

Why the de­lay?

It’s worth high­light­ing two con­tribut­ing fac­tors to this very im­pres­sive achieve­ment. Firstly, Ty­cho was very well-funded, and took data col­lec­tion un­prece­dent­edly se­ri­ously (note that this was all done be­fore telescopes!). Se­condly, Ke­pler de­serves great credit for his per­sis­tence in test­ing his the­o­ries against that data, and throw­ing them out when they failed to meet it. But there’s a much broader ques­tion of why it took al­most 2000 years from the first he­lio­cen­tric sys­tem to the dis­cov­ery of these laws, and even longer un­til ac­cep­tance of he­lio­cen­trism be­came wide­spread—why, in White­head’s words, “in the year 1500 Europe knew less than Archimedes who died in the year 212 BC.”

In short, the an­swer seems to be Plato and Aris­to­tle: as Koestler por­trays them, “two fright­ened men stand­ing in [Plato’s] Cave, fac­ing the wall, chained to their places in a catas­trophic age, turn­ing their back on the flame of Greece’s heroic era, and throw­ing grotesque shad­ows which are to haunt mankind for a thou­sand years and more.” This is un­duly harsh—and yet how dam­ag­ing the blind ad­her­ence to their ideas which lasted, with few ex­cep­tions, un­til the 16th cen­tury! Two of Plato’s most in­fluen­tial tenets: that true knowl­edge can­not be ob­tained by the study of na­ture, but must be gained by con­sid­er­a­tion of the perfect world of Forms and Ideas; and that, for meta­phys­i­cal rea­sons, all ce­les­tial mo­tion must be in perfect cir­cles at uniform speed. Aris­to­tle then in­tro­duced the dis­tinc­tion be­tween the im­perfect and change­able earth, and the eter­nal and un­chang­ing cos­mos; also, that each move­ment was due to an ob­ject’s telos, or pur­pose. Lastly, God was placed on the out­er­most sphere, fur­thest away from the Earth. Their com­bined in­fluence had as­tronomers think­ing in cir­cles for al­most 2000 years! And even the sci­en­tists whose work led to the end of these dog­mas were still some­what trapped by them: Ke­pler spent years try­ing to make Ty­cho’s data con­sis­tent with cir­cu­lar or­bits. Only when he felt that he’d con­clu­sively ruled those out did he move on to ovals and el­lipses.

More speci­fi­cally, Koestler iden­ti­fies five main ob­sta­cles to sci­en­tific progress be­tween the Greeks and the sci­en­tific rev­olu­tion. “The first was the split­ting up of the world into two spheres, and the men­tal split which re­sulted from it. The sec­ond was the geo­cen­tric dogma, the blind eye turned on the promis­ing line of thought which had started with the Pythagore­ans and stopped abruptly with Aristarchus of Samos. The third was the dogma of uniform mo­tion in perfect cir­cles. The fourth was the di­vorce­ment of sci­ence from math­e­mat­ics. The fifth was the in­abil­ity to re­al­ise that while a body at rest tended to re­main at rest, a body in mo­tion tended to re­main in mo­tion. The main achieve­ment of the first part of the sci­en­tific rev­olu­tion was the re­moval of these five car­di­nal ob­sta­cles.”

Gal­ileo and Newton

An early step for­ward was Ty­cho Brahe’s ob­ser­va­tion of a su­per­nova in 1572, and his demon­stra­tion that it wasn’t just a comet, but a new (and chang­ing!) phe­nomenon be­yond the or­bit of the moon. Then, soon af­ter Ke­pler pub­lished his first two laws in 1609, an­other blow against the Aris­tote­lean con­cep­tion of the uni­verse was struck by Gal­ileo’s ob­ser­va­tions of sev­eral moons of Jupiter, as well as ter­rain on the moon and phases of Venus. Th­ese were all ev­i­dence that the Earth was less spe­cial than had been thought. While Gal­ileo hadn’t in­vented the telescope, or been the first to no­tice the new moons us­ing one, he was the first to bring them to promi­nence in his book Side­real mes­sage. The book faced heavy crit­i­cism from other sci­en­tists, but even­tu­ally be­came ac­cepted af­ter Ke­pler threw his sup­port be­hind it.

This makes Gal­ileo’s biggest blun­der even more con­fus­ing: he sim­ply re­fused to ac­cept Ke­pler’s sys­tem of el­lip­ti­cal or­bits, and in­stead pro­pounded a ver­sion of Coper­ni­cus’ the­ory which nei­ther was el­e­gant nor matched the data very well. He also dis­missed Ke­pler’s cor­rect the­ory of moon-caused tides in favour of the clearly flawed hy­poth­e­sis that they were driven by the Earth’s mo­tion. This would have been unim­por­tant, if not for his in­sis­tence that the Church start rein­ter­pret­ing scrip­ture based on the ev­i­dent truth of Coper­ni­can he­lio­cen­trism, as shown by the ex­is­tence of tides.* Koestler at­tributes Gal­ileo’s per­se­cu­tion by the Church not to an in­evitable clash be­tween sci­ence and re­li­gion, but rather to Gal­ileo’s own bul­l­head­ed­ness in at­tempt­ing to force a the­olog­i­cal sur­ren­der with­out hav­ing much ev­i­dence, and his ten­dency to in­sult and alienate im­por­tant peo­ple. (A par­tic­u­larly dam­ag­ing ex­am­ple: in his Dialogue con­cern­ing the two chief world sys­tems, the views of Pope Ur­ban are put into the mouth of a dimwit called Sim­pli­cio.) Koestler ar­gues that the Church had a lot of re­spect for sci­en­tists, and in gen­eral was will­ing to rein­ter­pret doc­trine based on com­pel­ling sci­en­tific ar­gu­ments, but that Gal­ileo’s ac­tions made op­po­si­tion to he­lio­cen­trism last much longer than it oth­er­wise would have.**

Nev­er­the­less, we have Gal­ileo to thank for in­for­mal pre­cur­sors to New­ton’s laws of mo­tion: that ob­jects re­main at the same ve­loc­ity un­less acted upon by a force; and that (ig­nor­ing air re­sis­tance) ob­jects fall with the same con­stant ac­cel­er­a­tion re­gard­less of their weight. At this point, sci­ence was in a pivotal po­si­tion. Ke­pler’s laws ap­plied to ce­les­tial ob­jects; Gal­ileo’s ap­plied to ter­res­trial ob­jects. There was a grow­ing ac­cep­tance that the two were fun­da­men­tally the same, but no agree­ment on what force drove ce­les­tial mo­tion, or what planets would do with­out the in­fluence of the sun, or even what weight meant in a ce­les­tial con­text. Peo­ple be­lieved Ke­pler’s laws, but didn’t un­der­stand them: why el­lipses? Why an equal area? Ideas were also float­ing around to do with re­cip­ro­cal at­trac­tion be­tween ob­jects in a way that varied by dis­tance, but noth­ing con­crete—and the in­tro­duc­tion of mag­netism made peo­ple even more con­fused about which force might be do­ing what.

Then, of course, came New­ton’s grand syn­the­sis of grav­ity in 1687. Con­sider his thought ex­per­i­ment of firing a can­non­ball hori­zon­tally from a moun­tain. If fired slowly, it will fall to the earth quite soon. If fired faster, it will curve around the earth a lit­tle be­fore fal­ling. But if fired fast enough, it will con­tinu­ally “fall” to­wards the earth but never hit it: it will be in or­bit. New­ton was then able to use calcu­lus to de­rive the fun­da­men­tal rea­sons why Ke­pler’s laws must be true (for an in­tu­itive demon­stra­tion of why el­lip­ti­cal or­bits make sense, try throw­ing a mar­ble into a cone). Key to this was New­ton’s ac­cep­tance of grav­ity as “ac­tion at a dis­tance”, which had always been very con­tro­ver­sial. Apart from this, there were al­most no cases of two ob­jects af­fect­ing each other ex­cept by some medium phys­i­cally be­tween them. The one known ex­cep­tion, as men­tioned, was mag­netism—a helpful prece­dent for build­ing ac­cep­tance of New­to­nian grav­ity. Koestler on New­ton’s im­por­tance: “If one had to sum up the his­tory of sci­en­tific ideas about the uni­verse in a sin­gle sen­tence, one could only say that up to the sev­en­teenth cen­tury our vi­sion was Aris­totelian, af­ter that New­to­nian. Coper­ni­cus and Ty­cho, Ke­pler and Gal­ileo, Gilbert and Descartes lived in the no-man’s-land be­tween the two.”

Science as sleepwalking

As men­tioned in the open­ing quo­ta­tion, the book is called The Sleep­walk­ers be­cause that whole tran­si­tion was so con­fused and messy. Out of Coper­ni­cus, Ke­pler, and Gal­ileo, the first and last never let go of the Pla­tonic ideal of cir­cu­lar or­bits. And while Ke­pler’s laws were cor­rect, his ac­tual ar­gu­ments were rid­dled with mis­takes and con­tra­dic­tions. In ad­di­tion to the mys­ti­cism men­tioned above, one anec­dote is par­tic­u­larly tel­ling. Ke­pler be­lieved that el­lip­ti­cal or­bits had to re­sult from the com­bi­na­tion of two differ­ent forces: one link­ing a planet to the sun, and the other sim­ply act­ing on the planet it­self. This is ba­si­cally cor­rect: the first is grav­ity, which pulls planets to­wards the sun; the sec­ond is cen­trifu­gal force or in­er­tia, which pre­vents them from fal­ling in­wards. How­ever, in Ke­pler’s the­ory of physics these are ex­actly the other way around! He en­visaged the sun as pro­vid­ing a force which swept the planets around their ro­ta­tions, and a planet’s in­trin­sic mag­netism as what pul­led it to­wards or away from the sun. His “proof” of his 2nd law was an­other case of com­ing to the right con­clu­sion for the wrong rea­sons. Even New­ton, in the midst of his great triumph, had to ap­peal to God to ex­plain why grav­ity didn’t cause the whole uni­verse to col­lapse in­wards.

Over­all I think read­ing this book is helpful in un­der­stand­ing what it looks like to be very smart and also very con­fused. It’s also no­table just how much the lead­ing sci­en­tists in­volved were held back from pub­lish­ing by con­cerns about their rep­u­ta­tion amongst other sci­en­tists—and if the ones who we re­mem­ber now were so wor­ried about that, there were prob­a­bly many more who suc­cumbed to those wor­ries and who we’ve there­fore never heard of. Lastly, as Koestler high­lights sev­eral times, the em­piri­cal ar­gu­ments in favour of geo­cen­trism—like “why don’t we feel the Earth mov­ing?”, “why don’t we ob­serve Venus get­ting closer and fur­ther away?”, and “why don’t we see the stars shift­ing as we or­bit?”—were ac­tu­ally fairly so­phis­ti­cated and rea­son­able.*** And given the messi­ness of Coper­ni­cus’ sys­tem, there sim­ply wasn’t enough ev­i­dence to con­clu­sively de­cide in favour of he­lio­cen­trism at least un­til Ke­pler’s el­lipses—which were only dis­cov­ered be­cause Ke­pler had already de­voted his life to the hy­poth­e­sis. This sup­ports the Kuh­nian view of sci­en­tific rev­olu­tions as driven to a sig­nifi­cant ex­tent by per­sonal taste in paradigms. The Sleep­walk­ers was ac­tu­ally pub­lished just three years be­fore The Struc­ture of Scien­tific Revolu­tions, and in the epi­logue Koestler dis­cusses some very similar ideas:

“The philos­o­phy of na­ture evolved by oc­ca­sional leaps and bounds al­ter­nat­ing with delu­sional pur­suits, culs-de-sac, re­gres­sions, pe­ri­ods of blind­ness, and am­ne­sia. The great dis­cov­er­ies which de­ter­mined its course were some­times the un­ex­pected by-prod­ucts of a chase af­ter quite differ­ent hares. … There oc­cur in biolog­i­cal evolu­tion pe­ri­ods of crisis and tran­si­tion when there is a rapid, al­most ex­plo­sive branch­ing out in all di­rec­tions, of­ten re­sult­ing in a rad­i­cal change in the dom­i­nant trend of de­vel­op­ment. The same kind of thing seems to have hap­pened in the evolu­tion of thought at crit­i­cal pe­ri­ods like the 6th cen­tury BC or the 17th AD. … ‘In­tel­lec­tual progress’ has, as it were, lin­ear as­so­ci­a­tions—a con­tin­u­ous curve, a steadily ris­ing wa­ter level; whereas ‘evolu­tion’ is known to be a waste­ful, fum­bling pro­cess char­ac­ter­ised by sud­den mu­ta­tions of un­known cause, by the slow grind­ing of se­lec­tion, and by the dead-ends of over-spe­cial­i­sa­tion and rigid in­adapt­abil­ity.”

And fi­nally, a pre­scient pas­sage on the in­creas­ing power of sci­ence:

“Thus within the fore­see­able fu­ture, man will ei­ther de­stroy him­self or take off from the stars. It is doubt­ful whether rea­soned ar­gu­ment will play any sig­nifi­cant part in the ul­ti­mate de­ci­sion, but if it does, a clearer in­sight into the evolu­tion of ideas which led to the pre­sent predica­ment may be of some value. The mud­dle of in­spira­tion and delu­sion, of vi­sion­ary in­sight and dog­matic blind­ness, of mil­len­nial ob­ses­sions and dis­ci­plined dou­ble-think, which this nar­ra­tive has tried to re­trace, may serve as a cau­tion­ary tale against the hubris of sci­ence—or rather of the philo­soph­i­cal out­look based on it. The di­als on our lab­o­ra­tory pan­els are turn­ing into an­other ver­sion of the shad­ows in the cave.”


* One im­por­tant con­cept here is the differ­ence be­tween a model which matches ob­served data (thereby “sav­ing the phe­nomenon”) and a hy­poth­e­sis which is claimed to be liter­ally true. My im­pres­sion is that geo­cen­tric mod­els with epicy­cles were gen­er­ally con­sid­ered part of the former cat­e­gory, be­cause while they matched the data pretty well, there was no good ex­pla­na­tion of why planets would move as if they were on wheels at­tached to wheels, when the ex­is­tence of such wheels in the sky would be ab­surd. The Church also had no prob­lem with he­lio­cen­trism when pre­sented as the former (which Gal­ileo re­fused to do). Note that the ques­tion of when we should treat the­o­ries as de­scrip­tive mod­els or literal truth is still a key ques­tion in philos­o­phy of sci­ence (now un­der the head­ings of sci­en­tific anti-re­al­ism ver­sus sci­en­tific re­al­ism).

** Note that while Koestler’s schol­ar­ship gen­er­ally seems metic­u­lous, with co­pi­ous quotes from origi­nal sources, I’m still slightly skep­ti­cal about this ar­gu­ment—par­tic­u­larly af­ter read­ing the epi­logue, which dis­plays a pro-re­li­gion bias and cre­dulity to­wards ESP and time travel. See also this ar­gu­ment that even eas­ily-avoid­able cen­sor­ship is dan­ger­ous.

*** For an­other ex­cel­lent per­spec­tive on this, see Ja­cob Lager­ros’ es­say here.