For millennia everybody knew that human beings enjoy a privileged, unique position at the centre of the universe. That self-confidence began to crack after Nicolaus Copernicus suggested that the Earth goes round the Sun and an exciting but frightening possibility emerged: could life exist on other planets?
For the French astronomer Jérôme Lalande it was a no-brainer. After all, he explained, if you saw a flock of sheep in the distance you would never infer that some of them had stones inside their bodies instead of entrails as humans do. Similarly, since the planets resemble the Earth, they must be inhabited. But even for those who accepted that logical leap, uncertainties were multiplying as increasingly high-powered telescopes revealed more and more stars. Might other planetary systems exist? Could they also support life?
Like Lalande, many writers argued by analogy, a rhetorical technique that then punched considerable weight. Fuelled by scientific, religious and philosophical beliefs, discussions about the plurality of worlds – the expression used at the time – flourished throughout the 18th century.
Cosmic conversations
The debates went viral in 1686, when a young French poet called Bernard Le Bovier de Fontenelle published Entretiens sur la pluralité des mondes, a series of nighttime chats between an eloquent philosopher and a beautiful (if naive) marquise as they stroll beneath the stars. Even though Copernicus had shaken up the cosmos almost 150 years earlier, Fontenelle still felt obliged to hammer home the benefits of adopting a sun-centred model. Isaac Newton had not yet introduced his concept of gravity, and Fontenelle glorified his own national hero, René Descartes, whose clockwork universe was packed with swarming particles. Perpetually colliding with each other, they formed giant vortices of cosmic matter, each swirling around a central sun: our solar system was just one among multitudes stretching out across the cosmos.
Descartes had kept quiet about the possibility of extraterrestrial life, but Fontenelle deployed the same analogical tactic as Lalande, insisting that invisible planets circling distant suns must also be inhabited. His bestseller was translated into English by three different authors, including the playwright Aphra Behn. Infuriated by Fontenelle’s condescending depiction of the marquise, she silently altered his text, replacing any mention of men with ‘Men and Women’. She was also concerned about the absence of God. Imagine, declared the marquise’s didactic escort, that the cosmos is a giant theatre controlled from behind the scenes by an invisible puppeteer – but for English readers, Behn introduced a subversive preface protesting that Fontenelle ‘ascribes all to Nature, and says not a Word of God … one would almost take him to be a Pagan’.
Heavenly worlds
Like Descartes, Newton wrote virtually nothing on the topic. Even so, his followers interpreted this near-silence as confirmation, and the existence of inhabited planets soon became a standard tenet of Newtonianism. One of Newton’s leading propagandists, the Scottish mathematician John Keill, borrowed Fontenelle’s theatrical metaphor to teach his Oxford students ‘that all the Worlds or System of Worlds, are as so many Theatres’.
Unlike in France, God played a major role in British versions of the cosmos. Keill insisted that ‘we are to consider the whole Universe as a glorious Palace for an infinitely Great and everywhere present GOD’, while theologians and preachers turned to the Bible. Texts such as ‘In my Father’s House, are many Mansions, I go to prepare a Place for you’ lent themselves to the interpretation that ‘there are Worlds besides this whereon we inhabit; and it may justly be concluded, that they are inhabited by Beings who are far superior to us in Goodness’.
The existence of extraterrestrial life soon slipped from a debatable hypothesis into a fundamental tenet of Enlightenment culture. Astronomers repeatedly quoted the line ‘An undevout Astronomer is mad’ as if it were a scriptural text, although it came from Night Thoughts (1742-45), a long poem in which Edward Young set out to convert a libertine by conducting him on an imaginary tour of the heavens. Portraying an immense universe studded with worlds full of adoring worshippers, Young exhorted readers to marvel at God’s magnificence. Similarly, in his Essay on Man (1733-34) Alexander Pope reinforced the doctrine that God had created multiple inhabited worlds:
He, who thro’ vast immensity can pierce,
See worlds on worlds compose one universe,
Observe how system into system runs,
What other planets circle other suns,
What vary’d being peoples ev’ry star,
May tell why Heav’n has made us as we are.
Tautological arguments proliferated. John Adams, future president of the United States, explained that if the planets had had a slightly different initial velocity, ‘the Inhabitants would be tormented yea destroyed and the Planets left barren and uninhabitable Wastes by Extreme Vicissitudes of Heat and cold’. ‘Ergo’, he deduced, ‘an intelligent and benevolent mind’ had chosen the velocity they actually have. Robert Chambers’ Cyclopaedia fell into the same trap of circular logic, arguing that because the Moon is similar to the Earth, it must have rivers whose only use – supposedly – is to support terrestrial-style life.
Analogy and conjecture
Earth-centred cosmologies had assumed that the universe had been designed specifically for human beings, but Newtonian modernisers deemed that it was only ‘according to the old vulgar Opinion, that all things were made for man’. Many went still further, arguing that God’s major reason for creating so many stars was to provide more space for intelligent life to worship Him – whatever form that might take.
By leaning on analogy, the classicist Richard Bentley tried to reconcile the Earth’s new-found insignificance with the superiority of humans (or, men): ‘As the Earth was principally designed for the Being and Service and Contemplation of Men; why may not all other Planets be created for the like Uses, each for their own Inhabitants which have Life and Understanding?’ Other theologians emphasised that people should be humbled: Henry Baker wrote his poem The Universe (1727) expressly to ‘Restrain the Pride of Man’, insisting that ‘this Globe itself is so inconsiderable, so near to Nothing compared with the Grand Universe, that to … fancy himself therefore the Lord of the whole Creation, is as ridiculous, as it would be for the puny Inhabitant of an Ant-Hill, to strut about, and boast that all the Earth was made for him alone’.
Another approach was to postulate an Aristotelian Chain of Being that stretched up towards God. Relying on those handy argumentative devices ‘analogy and conjecture’, Soame Jenyns MP described the tiny gradations leading from stones and plants up through birds and animals to ‘the brutal Hottentot’, then ever upwards to reach first the terrestrial summit – ‘a Bacon or a Newton’ – and eventually ‘the inhabitants of other planets, to angels, and archangels’. Or as Benjamin Franklin explained, ‘the INFINITE has created many Beings or Gods, vastly superior to Man, who can better conceive his Perfections than we’.
The problem of Christ
By the end of the century, even the strongest telescopes had failed to provide irrefutable evidence of life elsewhere. The arguments continued, but now protagonists were using science to challenge Christian theology. Thomas Paine stated the case bluntly, protesting that if there are multiple worlds, why should Christ ‘quit the care of all the rest and come to die in our world, because they say one man and one woman had eaten an apple?’ The literary wit Horace Walpole spelt out his own predicament: ‘Fontenelle’s Dialogues on the Plurality of Worlds, first rendered me an infidel. Christianity, and a plurality of worlds, are, in my opinion, irreconcilable.’
One solution was to suggest that although God had made many worlds, only ours was cursed by original sin. In Messiah, Friedrich Klopstock describes Christ’s life on a corrupt Earth surrounded with countless worlds inhabited by innocent creatures. Another option was to argue that humans should express gratitude for being singled out. The Scottish moralist James Beattie decreed ‘that our fall and recovery may be useful to them as an example; and that the divine grace manifested in our redemption may raise their adoration and gratitude into higher raptures’.
In 1801 Edward Nares, a country rector and former Oxford Fellow, concluded ‘that those who have thought most soberly, and reasoned most coolly, have in few points been perfectly agreed. I am not so sensible of being able to decide any point in dispute’. The plurality of worlds debates provided wonderful opportunities to make definitive pronouncements without the inconvenience of providing solid evidence: after all, it was impossible to be proved wrong.
Patricia Fara is an Emeritus Fellow of Clare College, Cambridge. Her most recent book is Life after Gravity: The London Career of Isaac Newton (Oxford University Press, 2021).