The opinion that our World, or universe, is not the only one in the Whole, is attributed, in general terms, to many of the early Greek philosophers, notably to Anaximander. The exact meaning of a ‘Cosmos’, in this connexion, is perhaps not easy to fix. Aristotle is clear that the circle of the fixed stars is one and constant, but the author of the Stoical treatise on the Cosmos, found among his works, takes stars to be a part of the (one) Cosmos. An earth, such as ours with her atmosphere and moon, is essential, and a sun, or access to sunlight, and perhaps some planets. In the Dream of Scipio our solar system, with the earth in its centre, is described with great distinctness as a unit in space. The planets are always regarded as luminous points, stars somewhat out of place (see p. 268), possessing no definite magnitude or solid substance.

In theory the number of Cosmi might be infinite, but a shrinking from the vague ‘Infinity’, in later times associated with the Epicureans, led Plato, for instance, to restrict the number to a possible five. That he based this number upon that of the five regular solids may seem fanciful, but the solid angles and forms observed in crystals might reasonably suggest the hypothesis that the ultimate constituents of the crust of the earth would be found in the most perfect solid structures known to theory. In theory there is much that is attractive in these five solids. To one coming fresh from a study of Plane Polygonal Figures, which exist in infinite number, and, when regular, approximate more and more closely to the Plane Circle, it comes as a surprise to find that, in the next higher degree, the number of solid bodies so approximating to the Sphere is five only. Again, it seems almost a paradox that, of these five, the nearest approximation to the Sphere is attained, not by the body with twenty fine faces, but by that which shews only twelve, and those comparatively blunted and unshapely (pentagons). It was perhaps from such considerations that the Dodecahedron was held of special importance by the Pythagoreans. Plato’s study of the several faces of these solids, as available for construction or reconstruction of a world, leaves nothing to be desired, assuming that a solid body can be built out of plane figures, an assumption which appears to belong to the same habit of thought as that which makes the point the square of unity, and the lineal measure corresponding to the number two the first rectangle. As the pentagon defies the analysis available for the equilateral triangle or for the square, the Dodecahedron remains over, a model or pattern of a stitch-work world, as viewed from outside (Phaedo 110 B and Timaeus 55 C; see also Burnet’s Early Greek Philosophy, p. 341 foll.). It may not be amiss to be reminded that Kepler, mathematician as well as astronomer, spent many toilsome years in the endeavour to arrange the members of our solar system upon a plan based on the five solids. ‘If Kepler went out “to seek his father’s asses”, he found a kingdom, for it was in the course of these speculations, and through them, that he discovered not only his own “Third Law”, but also the truth, overlooked by Copernicus, that the orbit of each planet lies in a plane which passes through the centre of the sun.’ (Dreyer, Planetary Systems, p. 410.)

The discussion of the plurality of worlds, in the modern sense, begins with the very attractive work of Fontenelle, brought out, in its original form, in 1686, a year before Newton’s Principia, being a series of conversations between the Author and a witty and accomplished Marquise, as to the habitability of the several members of the solar system. The argument which followed is distinguished by many great names, those of Newton himself, Bentley, Huyghens, the Herschels, Dr. Chalmers, till it was brought to a head in the middle of the nineteenth century by Dr. Whewell and Sir David Brewster, writing respectively against and for the hypothesis. The subject was then one (as readers of Anthony Trollope will remember) upon which any one might be called upon to take a side in a London drawing-room. In more recent times interest has been concentrated upon Mars, who now possesses the distinction of having two satellites. We are only concerned to invite the reader to compare the religious argument addressed to the Stoics by Plutarch (p. 142 foll.) with the religious argument drawn by Dr. Chalmers and Sir David Brewster from the enrichment of the providential scheme for man upon our earth which would follow the conception of other earths tenanted by other beings perhaps of a higher order.

But it is natural that any such speculation should begin with the moon, and in fact we find the question of her habitability discussed by Theon and by Lamprias (pp. 293-9). With the later treatises on this subject, beginning with Lucian’s witty flight of fancy, we are not concerned. But an exception must be made for the very able works of Savinien de Cyrano, known to us as Cyrano de Bergerac, whose Histoire comique des États et Empires de la Lune appeared, probably, in 1650, and was followed by a similar work about the sun. Cyrano appears to be familiar with Plutarch: thus he meets in the moon the ‘daemon of Socrates’, who has also been the tutelary spirit of Epaminondas, of Cato of Utica, and of Brutus. The idea (due in the first place to Heraclitus) of being fed on smells, is worked out with much vivacity. But with so original and daring a writer, it is not quite easy to settle how much is due to any hint from others and how much to himself. A modern reader will not need to be reminded that Cyrano was not a person of whom it was wise to give an outspoken opinion in his lifetime. But I had wished to speak with nothing but respect of a man of real learning and genius, who, from whatever cause, did not bring to perfection any work worthy of himself.

See, on the general subject, an Essay by the late Professor Henry J. S. Smith in Oxford Essays, 1855.