In the comments on my last post, commenter ikkoki offered this quotation from Metrodorus of Chios (4th c. BC):
Also I remember that quote [from Xenophanes] coming up especially in conjunction with Metrodorus of Chios (4th century BC) of how “expecting life to exist only on Earth is like seeding a field and expecting only one plant to sprout” whenever alien life comes in the news.
Metrodorus has a wikipedia article, the quote I gave you before according to the English wikipedia is from Aëtius, Placita i.5.4. According to the Greek wikipedia he was first published in “Griechische Anthology”, Editions H. Stadtmüll, Leipzig 1894-1896 and he was translated in English in Loeb in 1915-1917.
A search in Google on the quote produces a number of versions. Popular Science in 1984 gives us this:
The belief that intelligent life might exist elsewhere in the universe … was not subjected to a truly scientific enquiry until recently. Yet it undoubtedly antedates recorded history and was stated as early as the fourth century B.C. It was then that Metrodorus of Chios wrote in his book On Nature that “to suppose that earth is the only populated world in infinite space is as absurd as to believe that in an entire field sown with millet, only one grain will grow.”
Several people reproduce the quote online. Others prefer this, from the Internet Encyclopedia of Science:
To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow.
As with so much that calls itself “scientific” online, no reference for the claim is given on that site.
By chance I now find “book 2 of Plutarch, On Nature” online here. In chapter 1 it says:
Pythagoras was the first philosopher that called the world [Greek omitted], from the order and beauty of it; for so that word signifies. Thales and his followers say the world is one. Democritus, Epicurus, and their scholar Metrodorus affirm that there are infinite worlds in an infinite space, for that infinite vacuum in its whole extent contains them. Empedocles, that the circle which the sun makes in its motion circumscribes the world, and that circle is the utmost bound of the world. Seleucus, that the world knows no limits. Diogenes, that the universe is infinite, but this world is finite. The Stoics make a difference between that which is called the universe, and that which is called the whole world;—the universe is the infinite space considered with the vacuum, the vacuity being removed gives the right conception of the world; so that the universe and the world are not the same thing.
In chapter 15 he also says:
Anaximander, Metrodorus of Chios, and Crates assign to the sun the superior place, after him the moon, after them the fixed stars and planets.
But I then find this book which tells me that “Pseudo-Plutarch (Aetius), Placita Philosophorum, reports that Anaximander, Metrodorus of Chios and Crates placed the sun highest of all, then the moon, then the planets and fixed stars.” Going back to the “Plutarch” and up a level here I find the work is in fact “Sentiments concerning nature with which philosophers were delighted”, which sounds like Placita Philosophorum to me, and “Text derived from The complete works of Plutarch : essays and miscellanies, New York : Crowell, 1909. Vol.III” which must be the Moralia or “ethical essays”, and can be found on Google books here. But if the Wikipedia article tells us that this work, book 1, 5:4 contains our quote, it must be here.
To Metrodorus it seems absurd, that in a large field one only stalk should grow, and in an infinite space one only world exist; and that this universe is infinite is manifest by this, that there is an infinity of causes. Now if this world be finite and the causes producing it infinite, it follows that the worlds likewise be infinite; for where all causes concur, there the effects also must appear, let the causes be what they will, either atoms or elements.
The interesting point about this is that Metrodorus is not expressing an opinion about life on other planets, as the original “quote” suggested. All he says is that if the universe is infinite then there must be multiple worlds.
It is interesting to discover Plutarch’s Moralia does exist in English, in five volumes, by Goodwin, and better yet that it is on Google books! It’s a mine of useful excerpts.