A dazzling multiplicity of worlds : summary of Fontenelle’s Plurality of Worlds (1686)

Helen De Cruz
11 min readJun 12, 2021

Frontispice of Fontenelle’s Entretiens Sur la Pluralité Des Mondes (1686)

Here I summarize an influential science-popularizing book by Bernard De Fontenelle, entitled Conversations on the Plurality of Worlds (1686), in French Entretiens sur la pluralité des mondes.

This post is based on an earlier Twitter thread that got retweeted a lot, and I hope this way to keep a bit more permanent record.

As background music, I recommend this subtle performance of Les Voix Humaines, Marin Marais (roughly of the same period) by Brandon Acker and Craig Trumpeter.

All quotes are from the 1990 Hargreaves translation. There are various earlier translations, as well as the French original, freely available online.

The Plurality of worlds is a series of five dialogues between a Marquise and a philosopher, set over five evenings (Fontenelle added a sixth evening in later editions, mainly with replies to objections, but I will disregard this and focus on the first edition, published in 1686).

It offers a disorienting, startling vision of a myriad of inhabited worlds. First, a quick review of the major themes.

Major theme 1. popularizing astronomy

In the late 17th century, astronomy was only beginning to come into its own as a discipline. It was still part of natural philosophy, i.e., that part of philosophy that deals with the natural world. The received scientific picture well into the 1600s was Ptolemy’s geocentrism, a picture where the universe was very small, with the Earth at its center, surrounded by planets, the Moon, and crystalline spheres with the stars.

Not 100 years before the publication of Plurality of Worlds, Giordano Bruno was burned at the stake for suggesting that there were multiple inhabited worlds. Galilei Galileo also had an unpleasant brush with the inquisition only 50 years ago. Although heliocentrism was significantly accepted in scientific circles, it was still relatively new.

Moreover, once heliocentrism became accepted, the implication becomes that our sun is just one among many. The sun is no longer at the center, but just comes one among many. If that is the case, might there be life on other planets that orbit other suns? This is the main puzzle presented in Plurality of Worlds.

Major theme 2. Education and emancipation of women

This is a series of dialogues between a Philosopher and a Marquise, while they stroll in her garden in the evenings and watch the stars. The viewpoint character (the “I”) is the philosopher. At the time, natural philosophy encompassed many scientific disciplines, including Astronomy, so the Philosopher is an obvious choice for the voice who instructs (and comments) on new astronomical insights.

However, the choice of the Marquise is less obvious. At the time that Fontenelle wrote, it was unusual to have a woman feature in a philosophical dialogue. Women were commonly thought to be inferior to men (in virtue, physical ability, intelligence). Moreover, dialogues were often presented between adversaries (see e.g., dialogues written by Hume or Galileo), where different characters would defend different positions. Here, however, the discourse is more akin to a discussion between a teacher and a brilliant, quick-witted student.

In 17th century France, a lot of discourse had turned openly misogynistic, deriding learned women as somehow silly or ridiculous. This was because men did not like the competition of women, especially wealthy female patrons of salons, in the public intellectual sphere. Several of these were patrons to Fontenelle. (see below for a pejorative cartoon from the early 19th century, deriding learned women as “bas-bleu” –from the English “blue stocking”, expressing the sentiment that a woman who reads and educates herself would quickly leave–the horror–her husband to do some of the childcare).

Basically, women were denied a lot of formal opportunities for education and if they wanted to educate themselves they were ridiculed for it. Not so by Fontenelle. His Marquise is clever, witty, a worthy conversation partner to the philosopher (the dialogue is written in first-person from the philosopher’s perspective), but also has not had any astronomy lessons before, so a completely blank slate about this.

Major theme 3. Science as emancipatory

Fontenelle aimed to write a book that was easy to understand and that had literary qualities, he refers to La Princesse De Clèves (a hugely influential novel, considered to be the first modern psychological novel) and hoped his book would have similar literary aspirations. The aim of the book is outreach and science popularization, so he wanted to be accessible, including to women who had hardly enjoyed any formal education.

Fontenelle believed science would emancipate us from being slaves of our passions, would free us from ignorance and prejudice, would help us realize how nature is marvelous and interconnected. These are all major themes in the Enlightenment and the Encyclopedistes later on.

He succeeded in his popularizing aims. The Entretiens sur la pluralité des mondes enjoyed enormous success. For instance, it was translated by the first female professional writer in England, Aphra Behn, just a couple of years after its publication in French.

Summary of the work

Now on to the Plurality of worlds itself. In the Preface to the work, Fontenelle explicitly makes the case for why a woman as interlocutor to the astronomy-knowledgeable philosopher.

The aims are emancipatory and pedagogical: “I thought this fiction would serve to make the work more enticing, and to encourage women through the example of a woman who, having nothing of an extraordinary character, without ever exceeding the limitations of a person who has no knowledge of science, never fails to understand what is said to her… Why would any woman accept inferiority to this imaginary Marquise, who only conceives of things of which she can’t help but conceive?”

Since Plurality of Worlds (spoiler) raises the possibility of alien life on myriad planets in myriad solar systems, Fontenelle must address some theological thorny issues. Did these creatures fall? They are, after all, not children of Adam. Fontenelle brushes these worries aside.

The first evening

The first evening starts with the Marquise and the Philosopher strolling in her garden in a pleasant evening breeze. As they watch the moon and stars, he raises the possibility that “every star could well be a world”.

He thinks that because it is pleasing, and pleasure is part of truth. The Marquise then asks him to share this truth. He first declines, saying it’s not enjoyment like comedy, she replies “Do you think I’m incapable of enjoying intellectual pleasures?” So he proceeds.

First, he offers an idea of what philosophy is. “All philosophy is based on two things only. Curiosity and poor eyesight”. If we had better eyesight (or more generally, less limited senses) we could simply peer up and see what these solar systems look like. But we can’t so we need philosophical speculation. If we lacked curiosity, we would not care about these solar systems.

He then likens nature to the back of an opera-house where you can see the mechanisms at work. The Marquise at first has some qualms about this mechanistic picture of nature, objecting that this is very mechanical, but later she says “Now that I know it’s like a watch, it’s superb that…the whole of nature is based upon such simple things”.

Then comes a discussion of why astronomy is such a great science to dabble in (the perfect pastime in the countryside), as well as an exposition of the Ptolemaic old geocentric view. To arrive at his vision of myriad inhabited worlds, Fontenelle eases the reader very gradually. First comes an exposition of Copernicus’ heliocentrism.

The Philosopher sings the praises of Copernicus, painting him as an iconoclast, as someone who put us in our place, away from the center of the universe.

He then explains how the Ptolemaic picture is all wrong and too complicated and explains Copernicus’ picture of sun, planets, the Earth just one of the planets and then comes a really charming, witty piece of dialogue that I include below to give a flavor of the work.

“You’ve forgotten the Moon,” said the Marquise.

“I’ll find her again,” said I. “The Moon turns around the Earth and never leaver her in the circle the Earth makes around the Sun. If she moves around the Sun it’s only because she won’t leave the Earth.

“I understand,” she said, “and I love the Moon for staying with us when all the other planets have abandoned us.”

Fontenelle, Plurality of Worlds, First evening.

The Philosopher explains that people clung on to Ptolemy because they want to put themselves at the center (of the universe), much like a courtier who tries to place himself in the most prominent position at court. The Marquise then objects: if that is right, how could we ever have accepted Copernicus?

The rest of dialogue is taken up with questions about the atmosphere, the speed of the Earth revolving around the Sun, that’s the end of the first Evening.

The second evening

Fontenelle eases the reader very gradually into his dazzling vision of multiple inhabited worlds, warming them up to the idea of alien life.

The second evening chiefly concerns the prospect of life on the Moon. He likens us looking at the Moon as Parisians looking at Saint-Denis from a distance (a northern suburb of Paris, currently), and not noticing any living person. They would certainly be wrong to conclude there were no people living in Saint Denis (pictured).

Fontenelle is not sure that there is life on the Moon because we don’t know enough about it. But it’s certainly possible. He then forwards the idea that lunar creatures would resemble Earth life, because nature creates both unity and diversity.

He compares this to the human face. There is a lot of diversity in faces but also a unified plan (individually, and across racial lines, there is quite some discussion of early race science in this book and it has aged OK esp compared to explicit racists like Hume and Kant).

The Marquise expresses sadness that Europeans will once be able to meet the Australians, but we will never be able to meet the inhabitants of the Moon because travel to it is impossible.

“And why,” I asked, “aren’t you disturbed about the inhabitants of that great land of Australia, which is still completely unknown to us? We’re passengers, all of us, on the same ship; they occupy the bow and we the stern. You see that the bow and stern have no communication, and that at one end of the ship they have no idea of what people are at the other…”
“Oh,” she replied, “I count the inhabitants of Australia as known because surely they must resemble us closely, and we’ll ultimately know them when we want to take the trouble to go and see them… but we’ll never know the people on the Moon, and that’s heartbreaking.”

Fontenelle, Plurality of Worlds, Second evening.

The third evening

The third evening continues discussion of life on the Moon. The Philosopher confesses there is not enough information to know whether the Moon indeed has life on it, but we will not know as we cannot reach it without our technology. However, the philosopher speculates

“What if they were skillful enough to navigate on the outer surface of our air, and from there, through their curiosity to see us, they angled for us like fish? Would that please you?”

“Why not?” She answered, laughing, “As for me, I’d put myself into their nets for of my own solution just to have the pleasure of seeing those who caught me.”

( Fontenelle, Plurality of Worlds, Third evening.

Then we go on to Venus. Here, the philosopher builds on the principle of “Why not?” Why could there not be life on Venus? The Marquise objects: “Always by saying “Why not” are you going to put people on all the planets for me?”

The Philosopher says that, yes, we can populate all the planets using this principle. The Marquise objects that it’s hard to imagine all these alien life forms “My imagination is overwhelmed by the infinite multitude of inhabitants on all these planets, and perplexed by the diversity one must establish among them”.

The Fourth Evening

The Marquise and the Philosopher speculate on whether the “why not” principle could give life on Mercury or on the Sun.

They find that Mercury is possible (though very hot) but the Sun is impossible to hold life, as it is covered with “a million Mount Etnas put together,” which all “vomit flames”.

The fifth evening

We’ve left the solar system and the Marquise and the Philosopher speculate on the possibility of life in other solar systems. When realization dawns, the Marquise expresses a sentiment very similar to Pascal: “here is a universe so large that I’m lost, I no longer know where I am, I’m nothing. Each star will be the center of a vortex, perhaps as large as ours? … As many spaces as there are fixed stars? This confounds me–troubles me–terrifies me.”

However, the philosopher’s response is “This puts me at my ease. When the sky was only this blue vault, with the stars nailed to it, the universe seemed small and narrow to me; I felt oppressed by it. Now… it seems to me I breathe more freely, I’m in larger air”

In the final pages of this short work, the Philosopher and the Marquise discuss the Milky Way and the realization that it consists of thousands, millions of stars.

Just like the Moon resembles Earth (see earlier Evenings), the Milky Way’s stars each resemble our sun. They then discuss intriguing idea of stars coming into being and dying. Not long before, astronomers believed stars were eternal and unchanging, but the Philosopher argues that the “universe could have been made in such a way that it will form new suns from time to time.” He offers the analogy: Imagine a gardener walking among the roses. Since roses live so briefly, each rose would say “Our gardener is eternal and unchanging”, and they’d feel justified because no rose would’ve .

In the same way, the ancients believed the stars eternal. Fitting to the flirtatious and philosophical atmosphere throughout the discourses, the final evening ends as follows:

“You’ve arrived at the last vault of the heavens, and to tell you if there are more stars beyond that, one would have to be more able than I am. You may put more systems there or not, it’s up to you. They’re properly the province of the philosophers, those great invisible countries that may be there or not as one wishes, or be whatever one wishes. I’m satisfied to have taken your mind as far as your eyes can see.”

“Well!” She cried. “I have the whole system of the universe in my head! I’m a scholar!”

“Yes,” I answered, “you are, well enough, and you’ve the advantage of being able to believe nothing at all of what I’ve told you, whenever you choose. I only ask of you, as payment for my trouble, that you never look at the Sun, the sky or the stars, without thinking of me.”

(Fontenelle, Plurality of Worlds, Fifth evening).