ET LES DEBUTS
DE LA SCIENCE MODERNE
David M. K nig ht (G reat Britain)
UNIFORMITY AND DIVERSITY OF NATURE
IN 17th CENTURY TREATISES ON PLURALITY OF WORLDS
Arm chair, a priori, science has its attractions, for its p u rsu it does not involve th e tedium , and perhaps even social stigma, of trav ail in the laboratory. B ut this kind of science has its difficulties too. The discoveries m ade thereby often tu rn out to be p u rely linguistic; and unanim ity as to the axioms and the m anner of their application seems impossible to ach ie v e.1 In th e ordinary affairs of physics and chem istry the advantage has lain w ith the sooty empirics, who have based th e ir hypotheses on experim ent and observation, or a t least tested them thereby; intrusions of a priori assum ptions have tended here to play a relatively m inor role, a t least in th e presentation of theory. B ut th e re are some fields w hich are inaccessible to observation, w here if th e re is to be science a t all, it m ust perforce be of the a priori kind. P robably the m ost popular of these from the seventeenth century on w ards has been the plu rality of worlds. Into this boderline te rrito ry betw een scientific knowledge and science fiction, a region w hich is only now beginning to be actually explored, a n um ber of im po rtan t figures in seventeenth century science boldly intruded, w ith analogy th e ir only guide. The stud y of th e w orks they w rote is both en tertain ing in itself, and may also cast light on the assum ptions they made elsew here; for in discussions of p lu rality of worlds the m etaphysics is often explicit, w hich in m ore form al treatises is suppressed.
There w ere various forms of this literatu re, some m ore genuinely concerned w ith th e physical conditions of th e various heavenly bodies th a n others. C yrano de Bergerac, for ex am p le,2 intended satire on E arth-dw ellers ra th e r than description of th e countries on the Sun and
1 For a discussion of this, see R. Harre, The A n ticip a tio n of N ature, London 1965.
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Moon. In A ntiquity this kind of tale was w ritten by Lucian in his
Icaromenippus and True History; such works are precursors of Jona
than S w ift’s, and perhaps of m o dem science fiction, b u t do not really tell us any m ore about speculative astronom y th an G ulliver’s Travels tell us about geography. So w e shall not be fu rth e r concerned with them, b u t w ith those who w ould have considered th a t they w ere doing w ork of scientific value.
P lu tarch had presented a rg u m e n ts3 in favour of th e Moon being another earth: “as an earth she seems perfectly beautiful, noble, and well ordered thing, b u t as a s ta r or lum inary or a divine and heavenly body, I fear she w ill prove unshapely and uncomely, and will do no credit to h er beautiful nam e.” Shining only by borrowed light the Moon gives us no heat; and like th e Earth, she has great m ountains, and depressions containing w ater or dark air, which, not being lit up by the Sun, appear dark and give rise to h e r ap parent face; the subject of the book. We should not suppose, in judging her a celestial earth, th a t she is: “a body w ithout soul and intelligence, and w ithout p art in the things of which it is m eet to offer th e first fru its to the gods...”; th at is, she does not lack w h at seventeenth-century authors were to call
“dress and fu rn itu re .”
The conjectures of P lu tarch w ere supported by the telescopic rese arches of Galileo; 4 who was able, from the shadows they cast, to esti m ate the height of th e lu n a r m ountains. The telescope, if one accepted w h at was seen through it as evidence, revealed th a t th e Moon was indeed another ea rth ra th e r th an a perfect sphere of quintessence; and also th a t Ju p ite r was encircled by moons, indicating an analogy be tw een th a t planet and ours. Galileo was him self relu ctan t to speculate on w h eth er there m ight be men on th e Moon, a notion w hich would have had theological dangers since all men w ere declared to be des- cendents of Adam and Eve, and no record existed of any em igration to the Moon. B ut K epler in his Dream, 5 a book w hich circulated for a long tim e in m anuscript, had already in 1609 described a “visit” to the Moon, and th e creatu res to be found there. L u n ar voyagers were drugged, and w hisked th ith er during an eclipse by daemons invoked by suitable incantations. K epler p u t seas on the Moon, and deep caverns to afford shelter from the ex trem e inclemencies of the climate. He spoke of the “people” on the Moon; b u t his main object was not to
3 In T. Heath, G reek A stron om y, London 1932, pp. 166—180. The quotations are from pp. 176 and 178.
/l See S. Drake; D iscoveries and O pinions of G alileo, N ew York 1957, pp. 21—58. And J. Kepler, C on versation w ith G alileo’s S idereal M essenger, trans. E. Rosen, N ew York—London 1965, p. 27.
5 J. Lear (ed.), K e p le r’s D ream , B erkeley—Los A ngeles 1965. Kepler appended P lutarch’s Face in th e Moon to this work; and had read Lucian. See also
describe these inhabitants, bu t to argue for the motion of the E arth by showing th a t M oon-dwellers (would like) us think of them selves as being at rest, w hile all the heavenly bodies circulated around them , and the E arth rotated before th e ir eyes.
In an appendix, w ritten later, K epler applied the doctrine of un ifor m ity of n atu re w ith some determ ination, arguing th a t geological p ro cesses analogous to those happening on E arth m ust sculpt th e lu n a r surface. B ut the round cavities there seemed artificial, th e resu lt of “arch itectural intellect;” and it was therefore necessary to conlude th a t th ere w ere rational creatu res who had built them. 6 These inhabitants m ust be very num erous in order to have b u ilt such constructions, w hich resemble, b u t surpass, in scale the P yram ids and the G reat W all of China. K epler suggested th a t since the lu n a r clim ate was m ore extrem e, and its landscape more rugged, than those w e encounter, th e inhabi tan ts would probably be bigger and h ard ier th an we. The round patches th a t they had so laboriously constructed could be nothing b u t fo rtifi cations against enem y assault.
K epler’s speculations about the Moon w ere at least based to some ex ten t on em pirical observations. The Moon afte r all could be inspec ted, and gross details on its surface descried, through telescopes. The solar planets, and th e ir satellites, could also be seen, although very few details could be discerned; so those w ho would p u t living creatu res on these heavenly bodies needed to take little note of em pirical evidence in th e ir imaginings. The next step was to postulate in h abitants for the hypothetical planets circling the fived stars; bodies whose existence rem ains unconfirm ed to this day. N aturally in this field the w ay lay open to the most unguarded speculation; fo r alm ost any properties could be proposed for the hypothetical inhabitants of hypothetical worlds. We should be surprised therefore a t the relative unanim ity of w riters on' th e subject ra th e r than at th e ir divergences.
In A ntiquity, the doctrine of p lu rality of w orlds was held by the Atomists, who, since they postulated an infinite num ber of atom s in an infinite void, believed there m ust be innum erable w orlds coming into being and passing aw ay at any given moment. All these worlds w ould be different; 7 according to Democritus: “th ere are innum erable worlds, which differ in size. In some w orlds there is no sun and moon, in others they are larger than in our world, and in others more num e rous.” The spacing of w orlds is irreg u lar also; and some are devoid of plants, moisture, or living creatures. The physicists of the seventeenth century, being more teleologically minded, accepted the thesis th a t
11 K e p le r’s Dream , p. 173; Kepler, op. cit., p. 28.
7 G. S. Kirk, J. E. Raven, The P resocratic Philosophers, Cambridge 1957, p. 411.
6 4 D. M. K n ig h t
th e re w as a p lurality of worlds, b u t denied the infinite diversity of natu re. A nother half-w ay position is found in P lu ta rc h ’s W hy the
Oracles Cease to Give Answ ers, w here th e suggestion is m a d e 8 th a t
though belief in an i n f i n i t e num ber of worlds arising by chance is incom patible w ith belief in God, the existence of several is very probable. Five was a likely num ber, since th ere w ere five Platonic solids; a speculation rem iniscent of K epler’s theory th a t the in ter plan etary distances could be accounted for in term s of a nest of these solids, centred on th e Sun.
Those who w rote on plu rality of worlds w ith whom we shall be most concerned here are Wilkins, perhaps the leading figure in the foundation of the Royal Society; Fontenelle, who became P erpetual S ecretary of th e French Academy of Sciences; and Huygens, one of the outstanding scientists of the day, best know n for his work on clocks, his astronom ical discoveries, and his wave theory of light. Speculations such as theirs n atu ra lly gained plausibility from Copernican astronomy, in w hich the E arth was one plan et among m any rath e r th an the centre of th e Universe; b ut in fact pre-Copernicans, applying th e principle of p le n itu d e,9 h ad concluded th a t if to create one w orld were good, to create several w ould be better; and th a t God would therefore have done so. Wilkins, who despite his m arriage to Cromwell’s sister be cam e a bishop a fte r the R estoration of Charles II, was concerned in his book 10 chiefly to m ake the w orld safe for Copernicans and speculators on p lu rality of worlds. Following the Baconian program m e of confining theologians to theology, he showed at lenght w h at absurdities were generated w hen th e statem ents of the F athers, and some passages of the Bible, w ere taken seriously as physics. “T ru th s” he noted, “have been form erly esteem ed ridiculous, and g reat absurdities entertained by common consent.” Aquinas had argued th a t other worlds m ust be the same or different; if the same, then w hy should have God made them ; and if different, then both cannot contain universal perfection, so n eith er can separately be a tru e w orld or universe. Wilkins tu rn s this by not using “w orld” in th is sense, b u t to mean “e a rth ” ; and his successors did the same. 11
The fact th a t the Bible did not m ention other worlds did not make any difference on W ilkins’ view, for the negative auth o rity of Scrip tu re is not “p rev alen t” in m atters not fundam ental to religion; besides,
8 A. H. Clough, W. W. Goodwin, P lu tarch ’s L iv es and W ritin gs, 10 vols., London, n. d., IX, pp. 29—39.
9 A. O. Lovejoy, The G reat Chain of Being, N ew York 1960, p. 115.
10 (J. W ilkins), The First Book. The D iscovery of a N ew W orld or, a Discourse ten din g to pro ve, th a t ’tis probable th ere m a y be another h abitable W orld in the Moone. W ith a discourse concerning th e p o ssib ility o f a Passage th ith er. 3rd imp., London 1640.
the solar planets are not even m entioned in the Book of Genesis as being created. In general, W ilkins declared, 12 “...absurdities have fol lowed, when men looke for the grounds of Philosophy [Science] in the words of S criptu re.” It would be more economical of Divine Wisdom, given a body such as the Moon, to use it both as a moon and a world, th an in only one capacity. The spots and b rig h t p arts on th e Moon seem a deform ity; b u t if it is a world, then they will be seas and land; and lu n a r soil is probably very like terrestrial. In its eclipsing of stars the Moon seems to give some evidence th a t it has an atm osphere, or “orbe of grosse vaporouse air,” and this is supported from analogy, for the sun also seems to have one. Probably the “m eteors” on the Moon— its w eather—would be like ours. P lu ta rch had thought such things m ight be very different, because of the variety of ways N ature uses to bring about sim ilar effects; b u t W ilkins argued th a t the close p a ra l lels between the E arth and the Moon revealed by the telescope in dicated a general sim ilarity.
In the same way, the ex ten t of positive analogies betw een the E arth and the other solar planets made it likely th a t all w ere worlds. In particular, S atu rn and Ju p ite r had moons; and W ilkins concluded th a t: “if you consider th e ir quantity, th e ir opacity or these other dis coveries, you shall find it probable enough, th a t each of them may be a severall w orld.” 13 As to th eir inhabitants, it is clear th a t P ro vidence has so furnished th e Moon w ith conveniences th a t th ere m ust be somebody to enjoy them. Probably not m e n ,14 “b u t some other kind of creatures, which beare some proportion, and likenesse to our n atu re s.” On the other hand, they m ight be quite different, for God m ight have glorified him self in the creation of an infinite diversity of creatures. P lan e tary creatures m ight be m idway betw een men and angels. W ilkins sanguinely expected th a t posterity would get to the Moon to converse w ith its inhabitants, using some kind of flying chariot to escape from the E a rth ’s gravitational force, w hich in W ilkins’ view extended only tw enty miles from the surface.
In a la te r book he tried to deal w ith the teleological or quasi- -aesthetic argum ent th a t the fixed stars were, on Copernicus’ hypo thesis, unnecessarily far away. His solution was th a t there w ere ratio nal creatures nearer these lum inaries th an we are: “our disabilitie to com prehend all those ends w hich m ight be aimed a t in th e w orks of nature, can bee no sufficient A rgum ent to prove th e ir superfluitie. Though S cripture doe tell us th a t these things w ere made for our use, y et it do’s not tell us, th a t this is th e ir only end. ’Tis not impossible,
12 Ibid., pp. 29, 37. See also G. M cColley, “T he S eventeenth Century Doctrine of a Plurality of W orlds,” Annals of Science, I, 1936, pp. 385—430.
53 Ibid., p. 180. 14 Ibid., p. 190.
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b u t th a t there m ay be elsewhere some other inhabitants, by whom these lesser stars may be more plainly discerned.” 15
In W ilkins’ works the lines are laid down along which Fontenelle and H uygens w ere to proceed; b u t w hereas in the 1640s it w as still necessary to defend the Copem ican system, by the 1680s this theory had become the accepted one. The au th o rity of the literally intepreted w ords of Scripture or the F athers was no longer, at least in astronom y, w h at it had been; and the only theologically dangerous area was spe culation on the inhabitants fo th e celestial worlds.
W ilkins had included the cavet th a t w hile the inhabitants of the Moon and th e solar planets w ere probably very sim ilar to those of th e Earth, they m ight be extrem ely, or even unim aginably, different. B ut if they were supposed altogether dissimilar, his whole argum ent would collapse, since he was arguing from the observed sim ilarity between the E arth and the Moon to a likeness in unobserved characteristics. No such observations and analogical reasonings could reveal anything about unim aginable entities. The argum ent th a t other planets m ight m erely be ra th e r sim ilar to the E arth seems at first sight a sensible solution; b u t one ought in proposing this to have some idea w hat the difference m ight be. In th e seventeenth century th e m ain diversities proposed w ere in climate, and in appearance of the heavens a t night; both these would be effects simply of the various distances of the planets from the Sun. The implication is therefore th a t the solar planets are all extrem ely similar, b u t differently situated. B u r n e t16 added the idea th a t they m ight be at different stages of parallel geological histories. The suggestion th a t the rational inhabitants of other w orlds m ight n ot be men was essentially verbal, advanced purely to avoid difficulties w ith the Churches. None of these escapes A quinas’ question, why th e re should be several w orlds w ith no essential difference between them ? In the nineteenth century, George Wilson did propose a possible chem i cal difference; there was no sufficient reason w hy the chemical elem ents should be unevenly distributed throughout the universe, and since on E arth some are rare, in other systems they m ust be com m on.17
Fontenelle w as anxious to propagate the Cartesian system of astro nomy as well as the doctrine of p lu rality of worlds. 18 Unlike many of his predecessors, he believed th at the Moon’s surface was not sui
15 (J. W ilkins), A Discourse Concerning a N ew Planet. Tending to prove, T h a t ’tis probable our Earth is one of the P lan ets. The second Booke, now first published, London 1640, p. 131.
16 T. Burnet, The Sacred T heory of th e Earth, reprinted, London—Fontw ell 1965, pp. 128—9.
17 G. W ilson, E lec tric ity and th e E lectric Telegraph, togeth er w ith th e C h em istry of the S tars, new ed., London 1895, p. 29.
18 B. le B. de Fontenelle, E ntretiens sur la P lu ra lité des M ondes, ed. R. S h ack - leton, Oxford 1955.
table for living creatures; though he was prepared to p u t some below the surface. Dwellers on other heavenly bodies w ould not, according to his account, be men, an d it would be absurd even to imagine th e ir characteristics; but he did so—after all, this is w h at his readers w ould have been interested in—and declared th a t th e ir tem peram ents fol lowed th e ir climate. Venusians w ere amorous, S aturnians and Jovians phlegmatic. The Sun, a self-lum inous body quite unlike the E arth, was not inhabited. Fontenelle evolved an interesting tw ist to the argum ent, in th a t he th rew onto his opponents th e onus of showing why the other planets should not be inhabited.
D uring the seventeenth century the universe became larger, in th a t among astronom ers belief in a sphere of fixed stars w as common a t the beginning of the century, w hile at the end th e received opinion was th a t the Creation was infinite, or at least indefinite. 19 D erham distin guished these two notions as the “Copernican” and th e “new ” systems. 20 Huygens was an adherent of the new system ; and using the teleological principle th a t everything m ust have been created for some purpose, and a principle of uniform ity of natu re, he set about peopling the cosmos. 21 He disagreed w ith those who installed inhabitants on the Sun and Moon, both because these seemed to be uncom fortable places and also because they had functions already; the one as a source of h eat and light, the other to illum inate the E arth and cause the tides. The moons of Ju p ite r and the fixed stars revealed by the telescope m ust have some purpose; it would be absurd to claim th a t they were created only to be so viewed. Ju p ite r m ust be inhabited; and these distant stars m ust be the centres of other system s of planets.
The p articu la r version of uniform ity principle used by H uygens was based on a notion of fairness, or of sufficient reason: “Now should we allow the P lanets nothing b u t vast Deserts, lifeless and inanim ate Stocks and Stones1, and deprive them of all those creatu res th a t more plainly speak th e ir Divine A rchitect, we should sink them below the E arth in Beauty and Dignity; a thing very unreasonable...” 22 The same argum ent established th a t the celestial worlds m ust contain rational creatures, and th a t they m ust be able to enjoy all our pleasures, and have made just as much progress in the arts of civilisation as we. If they w ere much sm aller th an we, they would have difficulty, for example, in using astronom ical instrum ents; and God m ust have given
19 A. Koyre, From the C losed W orld to th e In finite U niverse, N ew York 1958. 2" W. Derham, A stro-th eology, 10th ed., London 1767, p. X X XIV .
21 C. H uygens, The C elestia l W orlds D iscover’d: or C on jectu res Concerning the Inhabitants, Plants and Productions of th e W orlds in th e Planets, 2nd ed., London 1722. Sim ilar argum ents w ere used by the N ew tonian Richard B entley in his B oyle Lectures; reprinted in I. B. Cohen (ed.), Isaac N ew ton ’s P apers and L ette rs on N atural Philosophy, Cambridge 1958, pp. 356—360.
6 8 D. M. K n ig h t
them hands, since He has not w ithheld these useful organs from mon keys. Every planet m ust have the sam e cycle of rain, sunshine, eva poration, though each m ust have: “its W aters of such a tem per, as to be proportioned to its H eat.” 23
Anyone who was not an astronom er would be hard p u t to it to say w here among these celestial w orlds he was after a voyage through space; and indeed H uygens’ conception of the universe borders on the tedious. His successors have narrow ed the range of heavenly bodies w hich m ight be expected to be inhabited, and tend to allow, irrationally perhaps, for a little more variety; b u t the doctrine th a t other planets in the universe are inhabited by rational beings very sim ilar to us rem ains as pow erful as ever, especially among certain radio-astrono mers, who expect to exchange signals w ith these folk. The teleological argum ents of the seventeenth century have become less compelling; it m ust be the argum ent from sufficient reason—the pourquoi non of Fontenelle—w hich some among our contem poraries find so seductive.
23 Ibid., p. 28. For a fuller exposition of H uygens’treatise, see D. M. Knight, “C elestial Worlds D iscover’d,” The D urham U n iversity Journal, LVIII, I, 1965, 23—29.
