“You see that white part of the sky, called the milky-way. Can you guess what it is? An infinity of little stars, invisible to our eyes on account of their smallness, and placed so close to each other that they seem but a stream of light. I wish I had a telescope here to shew you this cluster of worlds.”
Bernard Le Bouyer de Fontenelle, Conversations on the Plurality of Worlds (1686)
Bernard Le Bouyer de Fontenelle was a French author and philosopher. He wrote history and theatre, and even tried his hand as a lawyer before quickly deciding the legal profession wasn’t for him. Fontenelle was the model of an Enlightenment man. He believed in the renewal of human will and reason, and argued with gusto against academic colleagues who thought the great works of the past could never be equaled.
In 1686, the Frenchman published Conversations on the Plurality of Worlds. Written as a string of exchanges between a philosopher (a thinly veiled stand-in for Fontenelle) and an intelligent woman called the Marchioness, the work established the template for Enlightenment extraterrestrial discourse by recasting astronomy as a question of humanity’s place in the cosmos.
The first evening starts with the Marchioness and the philosopher on an evening stroll. As they watch the Moon and stars, Fontenelle’s philosopher bashfully admits that “I have taken it in my head that every star may be a world”. His trepidation flows from the recognition that radical ideas often run against the grain of human nature, that people like to cling to that which flatters their pride.
Our philosopher explains that astronomers held on to the old Ptolemaic model of the heavens because they wanted to put themselves at the centre of the universe, much like the courtier who tries to place himself in the most prominent position at court. Copernicus won out, he tells us, because the Ptolemaic system buckled under the weight of its own complexity. When Mars appeared to move backwards in the sky, astronomers explained it by saying the planet circled on a little ring, which in turn circled on a bigger ring, and so on. These epicycles multiplied until the model eventually looked like a funhouse mirror version of Ptolemy’s original scheme. The reason we were hesitant to accept it is for fear of what a heliocentric account means for our place in the universe.
The Marchioness isn’t moved by his argument. She asks: “Do you suppose I feel humbler for knowing that the earth goes round the sun? I assure you I esteem myself just as highly as I did before.” This is the essential question of the book, one born of the Enlightenment confidence in reason and nature’s order. We only believed Copernicus, he says, because the system of nature compelled us to. Yet in doing so we learned to accept something that cuts against the human instinct to put ourselves at the centre of the cosmos.
Conversations on the Plurality of Worlds is a negotiation between science and the human condition. If the stars are other worlds, then the universe is richer than we ever imagined. That which seemed threatening — humanity’s downward movement in the celestial hierarchy — seemed to Fontenelle to demonstrate that reason could accommodate dislocation. He saw this upheaval as proof that reason could bear uncomfortable truths, that humans could (and should) draw dignity from their new place in the pecking order.
Is there anyone out there?
Today, speculation about aliens is old hat. We see it in popular media, in research efforts like Breakthrough Listen, and in academia where studies of extrasolar beings are finally approaching something like scholarly respectability. Whether you have a strong opinion on the matter or not, all of us are well aware of the possibility of life out there.
It’s easy to forget that wasn’t always the case. We take discussion about little green men for granted, but our ancestors probably wouldn’t understand the question. Then again, the preoccupation with ET certainly feels like a very modern phenomenon. It’s the stuff of Hollywood, pulp sci-fi magazines, and turn of the century novelists, isn’t it?
Aliens surely loom larger in our collective cultural imagination than ever before, but serious consideration of extraterrestrial life as we might understand it has been hundreds of years in the making. The roots of our moment begin with the convulsions of early modern science, when the telescope made the stars look like other suns and the planets look like other earths.
For most of human history the heavens were the realm of gods, spirits, and influences; only in the seventeenth century did people begin to ask, in a recognisably modern way, whether those distant worlds might harbour other kinds of life. Bernard de Fontenelle was one of the first to popularise that shift. In Conversations on the Plurality of Worlds, he wondered whether the planets of the solar system contained life, and even speculated that every star in the night sky contained a solar system like our own.
But he also knew that alien life may not look like us. On possible lunar inhabitants, he wrote: “I say there are inhabitants, and I likewise say they may not at all resemble us,” and that any alien life must adapt to its own planetary conditions: “when I affirm that the moon is not peopled by men; you will see that according to the idea I entertain of the endless diversity of the works of nature, it is impossible such beings as we, should be placed there.”
His modesty masked confident assertions about extraterrestrial existence based on the principle of plenitude, the belief that nature abhors waste and fills all possible spaces with life. What made Fontenelle’s case so effective was both the boldness of his claim and the elegance of its presentation. He wrapped unsettling ideas in polite conversation, choosing as his interlocutor a witty, curious woman. In doing so, he signalled that reason was not the preserve of scholars alone, but something that anyone with curiosity could exercise. As he explained:
“In these Conversations I have represented a woman receiving information on things with which she was entirely unacquainted. I thought this fiction would enable me to give the subject more ornament, and would encourage the female sex in the pursuit of knowledge, by the example of a woman who though ignorant of the sciences, is capable of understanding all she is told, and arranging in her ideas the worlds and vortices. Why should any woman allow the superiority of this imaginary Marchioness, who only believes what she could not avoid understanding?”
Plurality of words
Fontenelle wasn’t the only Enlightenment thinker wondering about alien life. The ‘plurality of worlds’ debate became a live issue as scientists continued to spy new solar objects down the end of their telescopes. Naturally, Enlightenment thinkers moved to grapple with the questions that flowed from these observations. Were humans unique in the cosmos? How would extraterrestrial life affect Christian salvation doctrine? What moral obligations might exist toward rational beings on distant worlds?
The Dutch polymath Christiaan Huygens' posthumously published Cosmotheoros in 1698, which provided a systematic scientific treatment of extraterrestrial life by applying Newtonian physics and observational astronomy. Unlike Fontenelle's accessible dialogues, Huygens wrote a dense tome that established methodological principles that would influence subsequent astronomical speculation.
His fundamental argument rested on the Copernican principle: “A Man that is of Copernicus's Opinion, that this Earth of ours is a Planet, carry'd round and enlighten'd by the Sun, like the rest of the Planets, cannot but sometimes think that it's not improbable that the rest of the Planets have their Dress and Furniture, and perhaps their Inhabitants too as well as this Earth of ours.” This style of inquiry, one that connected reason and analogy, became the dominant approach in Enlightenment extraterrestrial discourse.
Huygens provided remarkably detailed speculation about ‘Planetarians’ based on functional reasoning about intelligence and technology. He argued they must possess manipulative organs because “without their help and assistance men could never arrive to the improvement of their Minds in natural Knowledge.” Perhaps his most famous idea was that the inhabitants of Jupiter must cultivate something like hemp for rope-making in their sailing ships, an assumption that demonstrated the period's confidence in analogical reasoning and its assumption that technological development followed universal patterns.
Others discussed extraterrestrial visitors as a form of social criticism. Voltaire's Micromégas from 1752 features a giant from Sirius (Micromégas, 120,000 feet tall) who visits Earth with a Saturnian companion (a puny 6,000 feet tall) in a text that sought to provide a cosmic perspective on human vanity. When Earth's inhabitants claim the universe was created for their benefit, “the two travelers fell on each other, choking with laughter”.
Of course, no treatment of life amongst the stars would be complete without religion. American founding father Thomas Paine deployed extraterrestrial life as a central argument against traditional Christianity in The Age of Reason, published in three volumes between 1794 and 1807. Paine's core argument targeted Christianity's cosmic exclusivity:
“Though it is not a direct article of the Christian system, that this world that we inhabit is the whole of the habitable creation, yet it is so worked up therewith, from what is called the Mosaic account of the Creation, the story of Eve and the apple, and the counterpart of that story, the death of the Son of God, that to believe otherwise, that is, to believe that God created a plurality of worlds, at least as numerous as what we call stars, renders the Christian system of faith at once little and ridiculous, and scatters it in the mind like feathers in the air.”
Paine was not out to abolish belief in God, but he was out to reform it. If revelation on Earth was the only path to salvation, what of the innumerable other worlds? To posit a separate incarnation for each, he argued, was absurd; to limit salvation to Earth was parochial. Faith in the creator must reflect the immensity of creation, and Christian doctrine ought to accommodate the true scale of the universe.
Finally, Immanuel Kant integrated extraterrestrial speculation into his comprehensive cosmological system. His Universal Natural History and Theory of the Heavens from 1755 examined solar system formation, arguing that the same processes that produced life here would operate elsewhere throughout the universe.
In Critique of Pure Reason, he wrote: “if it were possible to settle by any sort of experience whether there are inhabitants of at least some of the planets that we see, I might well bet everything that I have on it. Hence I say that it is not merely an opinion but a strong belief (on the correctness of which I would wager many advantages in life) that there are also inhabitants of other worlds.”
The idea constituted a hierarchical arrangement of planetary inhabitants based on distance from the Sun. Beings on planets closer to the Sun would be of a denser and more refined nature, while those on distant planets would be made of lighter stuff. All would possess reason, but their physical forms and capabilities would vary according to planetary environments.
Kant used the possibility of extraterrestrials less to describe aliens themselves than to clarify what it meant to be human. For Fontenelle it was a way to charm readers into accepting displacement, for Huygens to prove the universality of nature’s laws, for Voltaire to puncture vanity, and for Paine to expose the limits of revelation. In each case, other worlds served as proxies for disputes over knowledge, power, and salvation.
New horizons
In Conversations on the Plurality of Worlds, the Marchioness tells the philosopher “You are making the universe so unbounded that I feel lost in it; I don't know where I am”. The proper response, he insists, is to feel the opposite:
“For my part, said I, I think it very pleasing. Were the sky only a blue arch to which the stars were fixed, the universe would seem narrow and confined; there would not be room to breathe: now that we attribute an infinitely greater extent and depth to this blue firmament, by dividing it into thousands of vortices, I seem to be more at liberty; to live in a freer air”.
Extraterrestrials were a rhetorical instrument. They allowed Enlightenment writers to weaken the idea of divine privilege and to argue for the universality of reason, law, and moral order. Speculation about other worlds was a way of imagining a universe without exemptions, a politics without ecclesiastical hierarchies, and a humanity defined by its participation in a community of rational beings.
In embracing the plurality of worlds, Enlightenment thinkers completed the Copernican revolution in the cultural imagination. Its ‘principle of mediocrity’ — the claim that Earth is not special, that what happens here is likely to happen elsewhere — was the scientific manifestation of the Enlightenment’s organising principle. Once you accept that the same laws of nature apply throughout the cosmos, you undercut the idea that anyone ought to benefit from a pre-ordained position on Earth.
Hello there fellow writer, I’ve been on Substack for around a month now.
I find your content quite interesting, and it appears on my feed often, so I thought you may like one of my articles.
This one is about Giants, and the evidence in early newspapers:
https://open.substack.com/pub/jordannuttall/p/giants-in-newspapers?r=4f55i2&utm_medium=ios
Enjoyable read