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The Extraterrestrial Life Debate, 1750-1900

The Extraterrestrial Life Debate, 1750-1900

by Michael J. Crowe

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"There isn't an uninteresting page in it. It is a masterly review of an intriguing subject, erudite and entertaining, clear and all-encompassing reading for anyone interested in 'one of the most wondrous and noble questions in nature' ― does extraterrestrial life exist?" ― New Scientist.
Are we alone in the universe? Are there other beings on


"There isn't an uninteresting page in it. It is a masterly review of an intriguing subject, erudite and entertaining, clear and all-encompassing reading for anyone interested in 'one of the most wondrous and noble questions in nature' ― does extraterrestrial life exist?" ― New Scientist.
Are we alone in the universe? Are there other beings on other worlds who gaze into the night sky and try to imagine us, as we try to imagine them? Those questions have been debated since antiquity, but it was during the Enlightenment that they particularly began to engage the interest of prominent scientists and thinkers. In this fascinating volume, Professor Michael Crowe offers the first in-depth study in English of the international debate that developed between 1750 and 1900 concerning the existence of extraterrestrial life, a problem that engaged an extraordinary variety of Western thinkers across the spectrum of intellectual endeavor. Astronomers such as Herschel, Bode, Lalande, and Flammarion all weighed in, along with French philosophers Rousseau and Voltaire, American patriot Thomas Paine, Scots churchman Thomas Chalmers, and a host of others. Professor Crowe gives them all their say, as they address the question as a point of science, as a problem of philosophy, as well as a religious issue. The book ends with the "discovery" by Schiaparelli of the canals of Mars, the expansion of the canal theory by the American astronomer Percival Lowell, and the culmination of the canal controversy with the demonstration of its illusory nature.
"Crowe's book is lucid and rich in historical detail. His analysis is so fascinating and his comments on the contemporary debate so pertinent that The Extraterrestrial Life Debate can be recommended for the thoughtful reader without reservation. While a model of scholarly analysis, it has the unusual virtue of reading with the excitement of high adventure." ― Sky & Telescope.

Editorial Reviews

This Dover edition is an unabridged, slightly corrected republication of the work originally published by Cambridge University Press in 1986. Crowe (philosophy of science, U. of Notre Dame) considers the testimony of astronomers such as Herschel, Bode, Lalande, and Flammarion; French Rousseau and Voltaire; American patriot Thomas Paine; Scots churchman Thomas Chalmers; and a host of other scientists, philosophers, and religious leaders. The book ends with the 19th-century controversy over canals on Mars. Annotation c. Book News, Inc., Portland, OR (booknews.com)

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The Extraterrestrial Life Debate 1750â"1900

By Michael J. Crowe

Dover Publications, Inc.

Copyright © 1999 Michael J. Crowe
All rights reserved.
ISBN: 978-0-486-14501-3


The Plurality of worlds debate before 1750: a background survey

1. The debate in ancient and medieval science and philosophy

The roots of the plurality of worlds debate as it existed in 1750 extend back to antiquity. The present chapter sets the stage in a summary fashion for the post-1750 developments by drawing on Steven J. Dick's recent history of the debate up to the time of Kant, as well as other relevant studies.

Regarding the existence of other worlds, the ancients of both Greece and Rome were deeply divided. Arguing the affirmative were the Epicureans, so called after Epicurus (341–270 B.C.), who developed certain ideas that had originated with Democritus and Leucippus two centuries earlier. Among the theories that we today consider most modern are (1) that . matter is composed of atoms, (2) that the present state of nature is the result of a long evolutionary process, (3) that life exists elsewhere in the universe, and (4) that there is no God, or at least no personal God. Modern though these ideas may seem to us, they all indisputably date from antiquity, where they can be seen in Epicurus's "Letter to Herodotus," in a passage in which the philosopher's atheism is implicit:

... there are infinite worlds both like and unlike this world of ours. For the atoms being infinite in number ... are borne far out into space. For those atoms ... have not been used up either on one world or on a limited number of worlds, nor on all the worlds which are alike, or on those which are different from these. So that there nowhere exists an obstacle to the infinite number of worlds.

Later in the letter he adds that "we must believe that in all worlds there are living creatures and plants and other things we see in this world...."

In reading these passages it is imperative to recognize that in them, as well as in most other ancient discussions, the term world had a meaning very different from the meaning we now give it. The Epicurean "innumerable worlds" were not the solar systems of remote stars; in fact, in Greek astronomy, the stars were typically thought of as located in a starry vault not greatly distant from the orbit of the outermost planet of our system. Rather, the postulated worlds of Epicurus were separate systems unseen by humans, each with its own earth, sun, planets, and stars. This fact shows that the basis of Epicurean pluralism lay not in direct observation but in the metaphysical materialism and atomism of its philosophy. Other worlds must exist because some of the chance conglomerations of infinite atoms in an infinite universe must form worlds, all things being possible. As Metrodorus of Chios, Epicurus's leading disciple among his contemporaries, put it, "It would be strange if a single ear of corn grew in a large plain or were there only one world in the infinite. And that worlds are infinite in number follows from the causes [i.e., atoms] being infinite." The passages from Epicurus and Metrodorus illustrate the Epicurean endorsement of an idea that Arthur Lovejoy called "the principle of plenitude," the doctrine that "no genuine potentiality of being can remain unfulfilled, that the extent and abundance of the creation must be as great as the possibility of existence and commensurate with the productive capacity of a 'perfect' and inexhaustible Source, and that the world is better, the more things it contains." For the Epicureans, this "Source" was infinite nature, whereas in later centuries some religious authors identified it with the omnipotent Creator-God.

The most influential proponent of Epicurean philosophy was the Roman poet Lucretius (ca. 99–55 B.C.), who in his De rerum natura blended elegant Latin verse with Epicurean ideas to explain topics ranging from the origin of language to optical illusion, from the sweetness of wines to the evolution and structure of the universe. Concerning this last topic, the poet asserts:

Granted, then, that empty space extends without limit in every direction and that seeds innumerable are rushing on countless courses through an unfathomable universe ..., it is in the highest degree unlikely that this earth and sky is the only one to have been created and that all those particles are accomplishing nothing. This follows from the fact that our world has been made by the spontaneous and casual collision and the multifarious, accidental, random and purposeless congregation and coalescence of atoms whose suddenly formed combinations could serve [to produce] ... earth and sea and sky and the races of living creatures.

The theological conclusion he draws from this doctrine is that "nature is free and uncontrolled by proud masters and runs the universe by herself without the aid of gods." After the rediscovery in the fifteenth century of Lucretius's poem, a host of authors from Gassendi to Newton and Kant investigated whether or not Epicurean atomism, evolutionism, pluralism, and atheism were detachable from each other.

Attacks on the materialist, atomist, and pluralist positions date at least from the period of Plato and Aristotle, both of whom opposed the claims of Democritus and Leucippus for the existence of innumerable worlds. In his Timaeus, Plato (428–348 B.C.) asserts that "there is and ever will be one only-begotten and created heaven," basing his claim on the arguments (1) that the uniqueness of the Creator implies the uniqueness of the creation and (2) that were the creation a composite, it would be subject to dissolution and decay. The writings of Aristotle (384–322 B.C.) contain an array of arguments against a plurality of worlds. In his On the Heavens he expounds his doctrine of natural place. Earthly and watery substances move downward, he explains, because they seek their natural place, the center of the earth, whereas air and fire move upward, again tending to their natural place. A piece of earth can, of course, be forced upward, but that is a forced, "violent" motion. The relevance of this doctrine is that Aristotle notes that were other worlds to exist, they would have to be composed, as is our own, of earth, air, fire, and water. Consequently, a portion of earth could be moving in natural motion in our world, whereas it would be in violent motion with respect to another world, an obvious contradiction. Another argument appears prominently in Aristotle's Metaphysics, where he explains the motion in our planetary system as due to the Prime Mover acting at its periphery. Were there more worlds than one, a plurality of Prime Movers would be necessary, an idea he rejects as philosophically and religiously unacceptable.

Although the plurality of worlds debate was most intense between the Epicureans and Aristotelians, other schools and individuals were involved. The Pythagoreans, for example, were reported to have believed that "the moon is terraneous, is inhabited as our earth is, and contains animals of a larger size and plants of a rarer beauty than our globe affords. The animals in their virtue and energy are fifteen degrees superior to ours, emit nothing excrementitious, and the days are fifteen times longer." Plutarch (ca. 46–120 A.D.) also speculated on lunar life in his De facie in orbe lunae, and Lucian of Samosata (ca. 120–200 A.D.) composed two fictional moon voyages. "Plurality of worlds" is in one sense ambiguous: It can mean either a number of simultaneously existing worlds or a succession of worlds in time. The Stoics favored the latter doctrine, the Roman statesman and orator Cicero (106–43 B.C.), for example, accepting it from them, while rejecting as absurd the idea of coexisting worlds, although he left open the possibility of life on the moon.

Early Christian scholars faced the formidable task of forging a viable intellectual tradition responsive to the sophisticated questions raised by Greek and Roman authors. To the idea of a plurality of worlds, their response was initially negative; in the third century, Hippolytus, for example, rejected it, as did Eusebius, bishop of Caesarea, in the fourth century and Theodoret, bishop of Cyprus, in the fifth century. Augustine of Hippo (354–430) did likewise, although he was more concerned to refute the Stoic doctrine of successive worlds. His opposition to both successive and simultaneous worlds, as well as to the principle of plenitude, is evident in his City of God:

For if they imagine infinite spaces of time before the world during which God could not have been idle, in like manner they may conceive outside the world infinite realms of space, in which, if any one says that the Omnipotent cannot hold His hand from working, will it not follow that they must adopt Epicurus' dream of innumerable worlds?

The possibility of a plurality of worlds was discussed in the thirteenth century as the Christian scholars of the West gradually gained access to the writings of antiquity. One of the most important of these scholars, Albertus Magnus (1193–1280), commented: "Since one of the most wondrous and noble questions in Nature is whether there is one world or many, ... it seems desirable for us to inquire about it." Such inquiry occurred and can be found in the writings of Michael Scot (d. ca. 1240) in Spain, William of Auvergne (ca. 1180–1249) in Paris, and Roger Bacon (1214–92) at Oxford. Albertus Magnus and his pupil Thomas Aquinas (1224–74) also added to the literature on the subject. All rejected a plurality of worlds - not a surprising result, given the enthusiasm of the period for the Aristotelian writings, from which, directly or indirectly, most of their antipluralist arguments came. This was certainly true of Thomas Aquinas, but with one important difference: As a Christian, he felt compelled to urge that the singularity of our world in no way contradicts belief in an omnipotent God. However, the finely wrought distinctions he employed to this purpose did not satisfy some of his contemporaries. In 1277 there occurred an ironic event that Pierre Duhem and others have claimed to be a major cause of modern science. The bishop of Paris, Etienne Tempier, pressed by theologians concerned that philosophers of Aristotelian inclination were championing doctrines that could be construed as limiting God's power, issued a condemnation of 219 such propositions. Included in the condemnation was proposition 34: "that the First Cause cannot make many worlds."

Abruptly after 1277 the milieu changed, with many authors formulating analyses aimed at showing that God could create multiple worlds. Although few urged that God did in fact do so, this process led to a valuable reexamination and critique of Aristotle's antipluralist arguments. Examples of this trend are Jean Buridan (ca. 1295–1358), rector of the University of Paris, and the Oxford-educated Franciscan, William of Ockham (ca. 1280– 1347), both of whom called into question Aristotle's argument based on his doctrine of natural place. The former urged that God could create worlds composed of other elements having their own natural places, whereas his more radical English contemporary relativized the notion of natural place by claiming that in different locales, the same elements would have correspondingly different natural places. An especially fascinating case is Nicole Oresme (1325–82), tutor to the future Charles V of France and eventually bishop of Lisieux, who in his translation of and commentary on Aristotle's De Caelo provided a brilliant critique of some of Aristotle's ideas. Admitting that no philosophical or scientific reasons would bar the possibility of a succession of worlds in time or a nested series of concentric worlds, he went on in considering the case of spatially separate worlds to urge that the motions of bodies were governed by their surroundings: For example, wood in water rises. As Dr. Dick puts it, "In a single stroke, with this definition Oresme transferred significance from the earth-outer sphere relation to a heavy body-light body relation independent of where the bodies were situated." Despite this and other criticisms of Aristotle's arguments, Oresme concludes that "... God can and could in his omnipotence make another world besides this one or several like or unlike it. Nor will Aristotle or anyone else be able to prove completely the contrary. But, of course, there has never been nor will there be more than one corporeal world...." The contrast, so striking in Oresme but present in other authors as well, between their openness to the possibility of God creating other worlds and their denial, that he did so cries out for explanation. Was it their reading of Scripture, fear of church authorities, doubts about whether or not pluralism could be reconciled with the Christian doctrine of the atonement, or other factors that caused Oresme's denial of pluralism? The problem is exacerbated by the fact that these late-thirteenth and fourteenth-century authors examined only the question of other worlds, not that of other inhabited worlds. There is little direct evidence that Scripture was a major factor. Aquinas had cited John 1:1–"The world was made by Him"–as a reason for believing that only one world exists, but this appears in his writings more as an undeveloped aside than as a substantive argument.

The cases of Nicholas of Cusa and William Vorilong shed light on the question whether or not fear of church authorities or tensions with the doctrine of the atonement influenced the debate. In 1440, Nikolaus Krebs (1401–64), commonly known as Nicholas of Cusa or Cusanus, published De docta ignorantia, a remarkable if also enigmatic masterpiece of the Middle Ages. Therein Cusanus endorses the idea of other inhabited worlds:

Life, as it exists on earth in the form of men, animals and plants, is to be found, let us suppose, in a higher form in the solar and stellar regions. Rather than think that so many stars and parts of the heavens are uninhabited and that this earth of ours alone is peopled–and that with beings, perhaps of an inferior type–we will suppose that in every region there are inhabitants, differing in nature by rank and all owing their origin to God, who is the centre and circumference of all stellar regions.

He even speculates on the nature of his extraterrestrials, prefacing his statement by the admission that such speculation is groundless:

Of the inhabitants then of worlds other than our own we can know still less, having no standards by which to appraise them. It may be conjectured that in the area of the sun there exist solar beings, bright and enlightened denizens, and by nature more spiritual than such as may inhabit the moon - who are possibly lunatics–whilst those on earth are more gross and material. (pp. 115–16)

Having populated the sun and moon, he adds: "And we may make parallel surmise of other stellar areas that none of them lack inhabitants, as being each, like the world we live in, a particular area of one universe which contains as many such areas as there are uncountable stars." (p. 116)

A superficial knowledge of the extraterrestrial life debate, including belief in the myth that Giordano Bruno was martyred for his pluralistic convictions, might lead one to suspect that these claims of Cusanus reveal a person with little sense of the politically acceptable, if not a man destined for imprisonment or burning at the stake. Nonetheless, as Pierre Duhem stressed:

When for the first time in Latin Christianity, one hears a person speak of the plurality of inhabited worlds, it is proposed by a theologian who a few years earlier had spoken at an ecumenical council; a person who in a very celebrated book sought to divine the characteristics of the inhabitants of the sun and of the moon, went on to be honored by the confidence of popes [and by] the most elevated ecclesiastical honors ...

All this is correct: Cusanus's political sensitivities were such that in 1437 he had been sent to Constantinople for the Council of Basel; moreover, eight years after his Of Learned Ignorance he was made a cardinal of the Catholic church. Although the issue of the reconcilability of pluralism with Christian conceptions of a divine incarnation and redemption was not treated by Cusanus nor, so far as is known, by the other authors who in the period after 1200 examined the idea of other worlds, this was done by the French theologian William Vorilong (d. 1463), who, after giving reason for believing that God could create another inhabited world, added:

If it be inquired whether men exist on that world, and whether they have sinned as Adam sinned, I answer no, for they would not exist in sin and did not spring from Adam.... As to the question whether Christ by dying on this earth could redeem the inhabitants of another world, I answer that he is able to do this even if the worlds were infinite, but it would not be fitting for Him to go unto another world that he must die again.

By the end of the Middle Ages, the challenge of modifying the Aristotelian system so as to accommodate a God who could create multiple worlds had in some measure been met. Further challenges were soon forthcoming. One of these came from the publication in 1473 of the recently discovered De rerum natura of Lucretius. With this, Western man was forced to confront another of the powerful philosophical systems forged in antiquity, one far less easily reconciled to Christianity. A still more formidable challenge appeared exactly seventy years later, when Copernicus claimed that our earth, rather than being the center of the universe, is but one of the planets. That claim, which before long some saw as giving legitimacy to the idea of a plurality of worlds, was nothing less than revolutionary.


Excerpted from The Extraterrestrial Life Debate 1750â"1900 by Michael J. Crowe. Copyright © 1999 Michael J. Crowe. Excerpted by permission of Dover Publications, Inc..
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