The question of whether it was possible that worlds other than our own could exist was one with which natural philosophers were preoccupied from Hellenistic Greece all the way through to the Copernican revolution, which concluded the debate for good. And it was a lively debate indeed -- beginning at the beginning in ancient Greece, on the side of multiple worlds stood the Atomists, like Lucretius and Epicurus; against them stood Aristotle, whose cosmology made it very clear that our world could be the only one.

For Aristotle and the Atomists, what it meant to be a world was an important consideration that underpinned both positions in the debate. In this case, world is synonymous with cosmos -- the totality of the universe. Where the Atomist cosmology allowed for more than one world/cosmos to exist, and indeed demanded it, Aristotle's system was considerably more stringent in the limitations it imposed: the movement of elements to their natural places in his cosmos did not allow for the existence of more than one.

With respect to what comprises the cosmos, Lucretius makes reference to "this earth and sky" (Lucretius, 65). His phrasing is telling, as not only does it define what the cosmos is (that is, earth and sky together), but it also implies that as there is a this there is also a that, another earth and sky, and perhaps more besides. This is indeed the case, as he goes on to set out the Atomist position on the plurality of worlds; it is made very explicit that there are as many worlds, as many cosmos, as can be imagined.

Atomist natural philosophy (particularly where it relates to multiple worlds) begins with the idea that the universe is unchanging and infinite, as "outside the universe there is nothing which could come into it and bring about [...] change" (Epicurus, 39). Everything in the universe is comprised of indivisible atoms, and the remainder is void (where there are no atoms); both of these are infinite in quantity, which a consequence of the boundlessness of the universe. This is also what allows for there to be multiple worlds: stemming from this boundlessness is the idea that "surely things must be wrought and effected" where there is room for it, and that "nothing in the universe is the only one of its kind" (Lucretius, 64). So following by all of these principles, it is certain that our world is not the only world.

Conversely, Aristotelian cosmology (and the physical principles that underlie it) does not make provisions for multiple worlds. The uniqueness of this world is a consequence of a much narrower and more detailed explanation of what the cosmos is than the one that the Atomists used. For Aristotle, it's made up of four elements, each of which behaves in a different way in terms of the way it moves: earth sinks toward the centre of the cosmos, with water atop it in another layer, air in turn above that, and finally fire in the topmost layer. The elements always seek to be in their rightful places; this is why when we drop a stone from a height it falls to the ground, or if we exhale underwater the bubbles rise to the surface, and is the reason that the earth is ordered the way that it is.

Suppose for a moment that there was another cosmos. It would have to be beside the one that we're in (i.e., from which we are observing it), otherwise it would interrupt the endless circular motion of the celestial spheres. And "all the worlds must be formed from the same types of body [i.e. elements]" (Aristotle, 276a30). In that case, the constituent elements of another world would behave in the same way as do those of this world. But then the movement of the elements in relation to each other for one cosmos would involve the elements of the one moving contrary to the way they ought to move in relation to the elements of the other cosmos, since they are spherical. And so it is that the plurality of worlds is an unfeasible proposition.

For natural philosophers the debate was a stalemate, but Aristotle's cosmology won out in terms of popularity. His texts were preserved by Islamic scholars and rediscovered by the west during the Middle Ages, and his position became the official party line of the Catholic church, with any dissenters looked upon with snideful disregard at best and religious fervor-fuelled wrath at worst.

Despite the resistance to astonomical innovation (and there was a good deal of resistance), mediaeval scientific thought was not entirely uncreative. Natural philosopher Nicole Oresme, writing well before the Copernican revolution when Atomism was an unpopular position to support, exemplifies this sly sort of inventiveness; in his commentary on Aristotle's On the Heavens, he counters the argument that says that there cannot be more than one world because of the way in which the elements move in relation to each other. Oresme's argument remains on Aristotle's terms but reaches the opposite conclusion -- that is, that there could indeed be more than one world -- which makes it an important argument in the plurality of worlds debate; it opened the floor to continued speculation on other worlds by demonstrating that multiple worlds could not be ruled out by the logical constraints imposed by the physical makeup of the universe.

The crucial passage, from his Livre du ciel et du monde, is this one:

But up and down are used otherwise with respect to heavy and light objects, as when we say the heavy bodies tend downward and the light tend upward. Therefore, I say that up and down in this second usage indicate nothing more than the natural law concerning heavy and light bodies, which is that all the heavy bodies so far as possible are located in the middle of the light bodies without setting up for them any other motionless or natural place. (Oresme, 550-51)

And the particular passage from Aristotle to which it responds, from Book I of On the Heavens, is this:

Therefore the parts of earth in another world are such as to move to the centre here and fire there towards the extremity of our world. Yet this is impossible: for if this happens, earth in its own world must move upwards, while fire must move to the centre, and similarly earth from this world must move from the centre naturally in moving to the centre in that world, because of the way in which the worlds are mutually positioned. For either we ought not to lay down that the simplest bodies in the many worlds have the same nature, or in saying that they do we must make the centre single, as well as the extremity; yet if this is so, there cannot be more than one world. (Aristotle, 276b11)

Oresme begins with the same principles: the heaviest element, earth, tends downward; fire, the lightest element, tends upward; water, heavier than air but lighter than earth tends downward less so than does earth, and air falls between water and fire in heaviness. But the way in which they move in relation to each other, each seeking to come to rest in its natural place, is where Oresme breaks away from Aristotle.

Rather than focusing on each element's rightful place in relation to the other elements, Oresme gives primacy to their tendencies to move so that the heaviest bodies are located in the middle, with lighter bodies arrayed around them. So there could indeed be two cosmos side by side, and each could still operate according to the principles of how elements move -- but rather than seeking out a natural place, as Aristotle said, they would simply move such that the heaviest elements were surrounded by the lightest elements, earth surrounded by water and air and fire. In this way there could be an unlimited number of worlds organised according to Aristotelian cosmology, each of them independent from the others.

The debate changed entirely with Copernicus and Kepler and the advent of the heliocentric model of the universe, replacing the earlier geocentric model. This created a distinction between world and cosmos that hadn't existed before, when the two were synonymous: now instead of the entire universe being comprised of the slice of earth and sky around the observer, it is only a single world. Most importantly, with the sun placed at the centre of the cosmos, the Earth lost the primacy that it had enjoyed in the past, not just leaving room for but encouraging scientific speculation on other worlds that was comparatively free from dogmatic restraints.

Aristotle. On the Heavens, ed. Leggatt. Aris & Phillips, 1995.
Epicurus. "Letter to Herodotus", trans. C. Bailey. Oxford, 1926.
Lucretius. On the Nature of the Universe. Penguin, 1994.
Oresme, Nicole. "The Possibility of a Plurality of Worlds", trans. Albert D. Menut. From A Source Book in Medieval Science, ed. Crant. Harvard, 1975.