The pre-Socratic philosopher Democritus (c. 460 – c. 370 BC) believed in an infinite universe, and therefore in an infinite number of worlds. Some might be bigger or smaller than earth, he concluded; some might have more moons or suns, some might have none—and some apparently would be very much like ours with versions of ourselves inhabiting them.
No actual texts by Democritus survive. As with other ancient authors, we have to piece together his philosophy from references and summaries in other authors’ works.
The third-century theologian Hippolytus, for example, gives this synopsis in his Refutation of All Heresies:
He maintained worlds to be infinite, and varying in bulk; and that in some there is neither sun nor moon, while in others that they are larger than with us, and with others more numerous. And that intervals between worlds are unequal; and that in one quarter of space (worlds) are more numerous, and in another less so; and that some of them increase in bulk, but that others attain their full size, while others dwindle away and that in one quarter they are coming into existence, while in another they are failing; and that they are destroyed by clashing one with another. And that some worlds are destitute of animals and plants, and every species of moisture. And that the earth of our world was created before that of the stars, and that the moon is underneath; next (to it) the sun; then the fixed stars. And that (neither) the planets nor these (fixed stars) possess an equal elevation. And that the world flourishes, until no longer it can receive anything from without.
This is, strictly speaking, a cosmological notion; for the truly philosophical implications, we can look to one of the texts that form the Hippocratic Corpus. In it, the author tells a story about Hippocrates being invited to Abdera to cure Democritus of insanity. He “has been made ill by the great learning that weighs him down.” The supposed symptoms of his insanity, however, might be better seen as the natural reaction of one who has realized that, since the number of worlds is infinite, all choices are made somewhere, all outcomes are possible, and all fortunes befall all of us—and thus we are freed from angst and worry. (Indeed, when the prototypical doctor meets the philosopher, he quickly concludes that he is not insane.)
Previously inattentive to everything, including himself, he is now constantly wakeful night and day, laughs at everything large and small, and thinks life in general is worth nothing. Someone marries, a man engages in trade, a man goes into politics, another takes an office, goes on an embassy, votes, falls ill, is wounded, dies. He laughs at every one of them, whether he sees them downcast and ill-tempered or happy. The man is investigating things in Hades and writes about them, and he says that the air is full of images. He listens to birds’ voices. Arising often alone at night he seems to be singing softly. He claims that he goes off sometimes into the boundless and that there are numberless Democrituses like himself. (trans. Wesley D. Smith)
Leonora Carrington: Crow Soup (1997)