I have remarked in the twentieth book of my Astronomy, that in every period of time it has been believed that the planets were inhabited, on account of their resemblance to the earth. The idea of the plurality of worlds is expressed in the Orphics, those ancient Grecian poems attributed to Orpheus (Plut. de Placitis Philosoph. l. 2, cap. 13.) Proclus has preserved some verses in which we find that the writer of the Orphics places mountains, men, and cities in the moon. The Pythagoreans, such as Philolaüs, Hicetas, Heraclides, taught that the stars were all worlds. Several ancient philosophers even admitted an infinity of worlds beyond the reach of our sight. Epicurus, Lucretius, and all the Epicureans were of the same opinion; and Metrodorus thought it as absurd to imagine but one world in the immensity of space, as to say that only one ear of corn could grow in a great extent of country. Zeno of Eleusis, Anaximenes, Anaximander, Leucippus, Democritus, asserted the same thing: in short, there were some philosophers who, although they did not consider the rest of the planets inhabited, placed inhabitants in the moon; such were Anaxagorus, Xenophanes, Lucian, Plutarch, (De Oracular. defectu. De Facie in orbe Lunæ,) Eusebius, Stobius. We may see a long list of the ancients who have treated on the subject, in Fabricius, (Biblio. Græcæ, t. 1. cap. 20.) and in the Mémoire de Bonamy (Acad. des Inscriptions, tom. ix.) Hevelius appeared as firmly persuaded of this opinion in 1647, when he talked of the difference between the inhabitants of the two hemispheres of the moon: he calls them selenitæ, and examines at length all the phœnomena observed in their planet, after the example of Kepler (Astron. Lunaris.) It was maintained at Oxford, in certain themes which are mentioned in the News of the Republic of Letters, June 1784, that the system of Pythagoras on the inhabitants of the moon was well founded: two years afterwards Fontenelle discussed this subject in his agreeable work. There are farther details of the different astronomical opinions at the end of Gregory's book. For the objections, we may refer to Riccioli. (Almagestum, tom. 1, p. 188, 204). In 1686 the Plurality of Worlds was adorned by Fontenelle with all the beauties of which a philosophical work was susceptible. Huygens (who died in 1695) in his book entitled Cosmothéoros, published in 1698, likewise enters largely into the subject.
The resemblance between the earth and the other planets is so striking, that if we allow the earth to have been formed for habitation, we cannot deny that the planets were made for the same purpose; for if there is, in the nature of things, a connection between the earth and the men who inhabit it, a similar connexion must exist between the planets and beings who inhabit them.
We see six planets around the sun, the earth is the third; they all move in elliptical orbits; they have all a rotatory motion like the earth, as well as spots, irregularities, mountains: some of them have satellites, the earth has one satellite: Jupiter is flattened like our world; in short there is every possible resemblance between the planets and the earth: is it, then, rational to suppose the existence of living and thinking beings is confined to the earth? From what is such a privilege derived but the groveling minds of persons who can never rise above the objects of their immediate sensations?
Lambert believed that even the comets were inhabited. (Systême du Monde, Bouillon; 1770.) Buffon determines the period when each planet became habitable, and when it will cease to be so, from its refrigeration. (Suplèmens, in 4to. tom. 11.) What I have said of planets that turn round the sun, will naturally extend to all the planetary systems which environ the fixed stars; every star being an immoveable and luminous body, having light in itself, may properly be compared with our sun. We must conclude that if our sun serves to attract and enlighten the planets which surround it, the fixed stars have the same use. It is thought that the sun and fixed stars are uninhabitable because they are composed of fire; yet M. Knight, in a work written to explain all the phœnomena of nature, by attraction and repulsion, endeavours to prove that the sun and stars may be habitable worlds, and that the people in them may possibly suffer from extreme cold. M. Herschel likewise thinks the sun is inhabited (Philos. Trans. 179. p. 155, et suiv.)
Some timid, superstitious writers have reprobated this system, as contrary to religion: they little knew how to promote the glory of their Creator. If the immensity of his works announce his power, can any idea be more calculated than this to exhibit their magnificence and sublimity? We see with the naked eye, several thousands of stars; in every part of the firmament we discover with telescopes, innumerable others; with more perfect telescopes, we still find a multitude more. We compute, from the number seen through Herschel's telescopes in one region of the sky, that there are a hundred millions. Imagination pierces beyond the extent of vision, beholding multitudes of unknown worlds, infinitely more in number than those which are visible to our sight; and ranges unrestrained in the boundless space of creation.
Our only difficulty with respect to the inhabitants of so many millions of planets, is the obscurity of the final causes, which it is difficult to admit when we see into what errors the greatest philosophers have fallen; for instance Fermat, Leibnitz, Maupertius, &c. in attempting to employ these final causes or metaphysical suppositions of imagined relations between effects that we see and the causes we assign them, or the ends for which we believe them to exist.
If the plurality of worlds be admitted without difficulty; if the planets are believed to be inhabited, it is because the earth is considered merely as a habitation for man, from which it is inferred that were the planets uninhabited they would be useless: but I will venture to assert that such a mode of reasoning is confined, unphilosophic, and at the same time, presumptuous. What are we in comparison of the universe? Do we know the extent, the properties, the destination, and the connexions of nature? Is our existence, formed as we are, of a few frail atoms, to be considered any thing when we think of the greatness of the whole? Can we add to the perfection and grandeur of the universe? These ideas are expressed by Saussure, who in speaking of a traveller to Mont-blanc says: "if, during his meditations, the thought of the insignificant beings that move on the face of the earth offers itself to his mind, if he compares their duration with the grand epochs of nature, how great will be his astonishment that man, occupying so small a space, existing so short a time, can ever imagine that his being is the only end for which the universe was created."
From these considerations d'Alembert, in the Encyclopedia, (art. World) after examining the arguments for supposing the planets inhabited, concludes by saying: the subject is enveloped in total obscurity.
But Buffon affirms that wherever there is a certain degree of heat, the motion produces organized beings; we need not enquire in what way, but imagine these to be the inhabitants of the planets: if that should be the case, we may conclude it highly probable that they are inhabited, notwithstanding the preceding objections.
LA LANDE.