Despite the powerful forces arrayed behind the Aristotelian world view, it took time to halt the intellectual momentum in favor of habitable worlds. The famous Roman poet Cicero was interested in the possibility of living beings on the Moon, and his Somnium Scipionis may have inspired Plutarch (46 A.D. - 120 A.D.) to write his account of a visit to the Moon. In Facies in Orbe Lunare, after dealing with various problems involved in reaching the Moon, the Greek historian endorsed the Pythagoreans thus: "They affirm that the Moon is terrestrial and inhabited like the Earth, peopled with the greatest living creatures and the fairest plants..." He continues:
It is possible that some inhabitants exist on the Moon; and those who claim that these beings must need everything that is necessary to us, have never considered the variety that nature offers so that animals differ amongst themselves more than they differ from inanimate life.
Only forty years after the death of Plutarch, the Greek satirist Lucian of Samosata (125 A.D. - 190 A.D.) wrote the first interplanetary romance that has survived the ravages of time. In his elaborate True History Lucian and his fellow travelers are carried by whirlwind to the Moon, found to be inhabited by a race of men who ride on the backs of three headed birds. The adventurers have arrived at a most inopportune moment, as the Lunarians are in the middle of a war with the inhabitants of the Sun to settle a dispute over the colonization of Venus. The space troops include such marvelous creatures as "Horse-vultures," "Salad-wings," and "Flea-archers" (archers astride giant lunar fleas).The story is reminiscent of the "space opera" of the 1930's and 1940's.
But after Lucian there was no further debate of the possibility of visiting other worlds and meeting the indigenous lifeforms there -- for more than a thousand years! This may probably be attributed to the pervasiveness of the Church philosophy and its rigid opposition to the idea of the plurality of worlds. The pronouncement of Franciscus Gratianus, Bishop of Chiusi, in 1145 A.D. was perhaps typical: The belief in many worlds was to be condemned as heresy.
Of course, there was a serious logical flaw in this stance. If God really was all-powerful, why was he only able to create one world? Conversely, if only one world existed how could God possibly be truly infinite and omnipotent? The theologian Thomas Aquinas (1225 - 1274) came up with a "solution" to the problem: God had the power to create infinite worlds, but all the matter in the universe had been used to construct Earth!
Despite the obvious holes in this reasoning, the Church subsequently partially reversed its extreme position. In 1277, under the authority of the Pope, the Bishop of Paris decried as new heresy the belief that a plurality of worlds was impossible! This did not, of course, means that the Church began to teach the plurality of worlds. According to the physics of Aristotle, still in vogue until the 16th century, if any other worlds did exist they would have to gravitate to the center of the universe (where Earth was). But it became wrong to suggest that God could not create many worlds if He wished.
The debate was far from ended. In 1410 the Jewish philosopher Crescas wrote: "Everything said in negation of the possibility of many worlds is vanity and a striving after wind." Still, he was unwilling to stick out his neck very far:
...yet we are unable by means of mere speculation to ascertain the true nature of what is outside this world; our sages, peace be on them, have seen fit to warn against searching and inquiring into what is above and what is below, what is before and what is behind...
The first really explicit deviation from orthodoxy occurred during the Inquisition in Europe in the mid-fifteenth century. Cardinal Nicolas of Cusa, Bishop of Brixen and Christian philosopher, wrote a book calledOf Learned Ignorance (1440) in which he stated:
Rather than think so many stars and parts of the heavens are uninhabited, and that this Earth or ours alone is peopled.. .we will suppose that in every region there are inhabitants, differing in nature by rank and all owing their origin to God.
Considering how little we know about other animals here on Earth, he claims, "of the inhabitants.... of worlds other than our own we can know still less, having no standards by which to appraise them." It is said that Cusa escaped the Inquisitional wrath only by virtue of his special protection and friendship with Pope Eugene IV.
As astronomical observations became more accurate, the geocentric Aristotelian/Ptolemaic world view began to generate problems that were difficult to resolve. Calculated positions of the planets, for instance, were invariably in error. This necessitated the concoction of elaborate "explanations" based on a kind of astronomical fudge factor.
During this time the first tale of interplanetary travel since Lucian (thirteen centuries earlier) was published. Ludovico Athsto's (1474-1533) Orlando Furioso tells of a trip to the Moon using a chariot driven by Saint John. The vehicle is drawn by flaming horses, who leap from the summit of a high mountain. The Moon, it turns out, is littered with cities and townships. The heavy theological flavor of the story may have helped save Ariosto from persecution.
A mere eleven years later the first edition of Copernicus‘ renowned De Revolutionibus Orbium Caelestium appeared, proposing the modern Sun-centered (heliocentric) solar system. If the Holy See was enraged at this they could do nothing, for the Polish astronomer died the year his book came out --
Others were not so lucky. Forty one years after the death of Copernicus a Dominican monk by the name of Giordano Bruno (1547-1600) wrote his controversial On the Infinite Universe and Worlds. Among other things, the Italian philosopher advanced the following heterodoxies: "Innumerable suns exist; innumerable earths revolve about these suns in a manner similar to the way planets revolve around our sun. Living beings inhabit these worlds."
Although Bruno was visiting in relatively tolerant Great Britain at the time his book was published, as soon as he set foot on Italian soil he was promptly arrested by the Church and incarcerated without trial for seven years. He was then convicted of heresy by a tribunal of the Holy See and sentenced to death. Bruno was burned at the stake in the Campo de' Fiori in Rome on February 17, 1600.
With the improvement of the telescope by Galileo (1564-1642) and the subsequent observations of the mountainous terrain of the lunar surface, it became clear that the Moon was quite similar to the Earth in many ways. His discovery of the four largest Jovian satellites confirmed the existence of many worlds. For his part in advancing the heliocentric Copernican astronomy and the hypothesis of the plurality of worlds, Galileo was arrested by the Inquisition and forced to recant his heresies. Luckily, he was not executed.
Johannes Kepler (1571-1630) further refined the Sun-centered cosmology by suggesting that planets move in ellipses rather than perfect circles. He also authored an engrossing fictional account of a trip to the Moon, published four years after his death, entitled Somnium. Lunar biology is described in some detail, including several forms of vegetation and serpentlike grotesque monsters.
The first narrative of a trip to the Moon written in English was penned by Bishop Francis Godwin in 1638. In The Man in the Moone the main character, Domingo Gonsales, uses a team of trained geese under harness to carry him to the Moon whereupon:
Suddenly I saw myself environed with a kind of people most strange, both for their feature, demeanure, and apparel. Their stature was most diverse, but for the most part twice the height of ours; their color and countenance most pleasing, and their habit such as I know not how to express....
By 1640 another book was out, a two-volume set by fellow English Bishop John Wilkins, entitled The Discovery of a World in the Moone. Wilkins asserted his straightforward belief "that it is possible for some of our posterity to find out a conveyance to this other world, and if there be inhabitants there, to have commerce with them."
The roadblocks to the idea of intelligent alien life on other worlds were rapidly disintegrating.