Ockham's Plurality of Worlds
Plurality should not be posited without necessity. -William of Ockham
William of Ockham Portrait1288 - 1349
Weblink: Philosophy Professor
Weblink: Philosophy Professor
William of Ockham is best known for his razor, which, in the shortest of terms, is that which is simplest is that which is right. However, his view on the plurality of worlds, or at least on the possibility of the plurality of worlds can seem quite the contrary to his own beliefs. Ockham spends a good portion of his Commentary on the Sentences of Peter Lombard answering the question of the plurality of worlds.
I say that God can make a world better than this one, which would be only numerically distinct from this world. My reason is as follows: God can make an infinity of individuals of the same species [kind] as the individuals existing today; He can therefore make as many or more individuals than those having been already produced, and He can make them of the same species. But God is not constrained to produce them in this world; He can produce them outside this world, and from them make another world, just as he made this world from those things which exist now. 
Ockham must fight against the centuries of Aristotelian physics which had been so ingrained in theological and metaphysical thought. Aristotle denied the possibility of other worlds vehemently:
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’s physics was based upon the idea that the entirety of the universe is made up of the four elements: water, fire, air, and earth. Fire always rises away from the center and earth always falls towards the center. Therefore, according to Aristotle, it is not possible for two worlds to exist because what is there to stop the earth(that is of the element) of the other world from falling to our world. Such action would be contrary to its natural movement, that towards its own world. 
The Spell Binder
Though Ockham counters this with the following:
If a thing gets farther from a place naturally, regardless of its initial position it will tend toward this place only by a violent movement. But if it gets farther from this place from certain initial positions only, it is not necessary that it always approaches it by a violent movement.
From this Ockham introduces his argument of the two fires:
The fire located between the center of the world and the circumference of heaven imparts an example; when it tends toward the nearer portion of this circumference, it strays away from the opposite side. If however one puts it between the center and this opposite side, it would tend toward it naturally. 
Furthermore Ockham argues against the belief that there cannot exist more than one world or multiple heavens, because our heaven contains all the matter required for its nature:
…heaven is made of all already existing suitable matter, but not of all matter that can exist. In fact, God can create again celestial matter in the same way that He can create a new quantity of matter of any kind whatsoever.
William of Ockham’s theory on the possibility of other worlds is based on a cosmological view that God is a necessary being. Ockham’s God is all powerful and as such his fracturing of matter in to multiple possible universes is entirely within the realm of possibility to Ockham. Ockham’s arguments against Aristotle are quite sound up until his requirement of an all powerful being. By his assumption that this being does exist without providing the sufficient reasoning his entire argument feels as though it falls a bit short. He is just one foul unrestrained step from unleashing the Flying Spaghetti Monster on us all. The simplest answer, and that which follows the idea of Ockham’s razor most closely, is that no there is only one world (or universe) and that we are in it. However, believing this merely because it is the simplest of all the possibilities would be both negligent and unintelligent. What is necessary for the entirety of this to become a sound argument is either a proof for the existence of god, which will not be found even in modern philosophy of religion, or an alternative to the need of a greatest possible being.
 William of Ockham. Commentary on the Sentences of Peter Lombard. 14th century C.E.
 Aristotle. On the Heavens. 276b11
 Pierre Duhem, Medieval Cosmology: Theories of Infinity, Place, Time, Void, and the Plurality of Worlds, ed. and trans. by Roger Ariew (Chicago: Chicago University Press, 1987)
1. Ockham's commentary was perhaps the most difficult of all primary sources to locate. Though it was translated by one of the most famous Latin to English writers of modern times, Ariew and Livesey, I was limited to the little bit that made its way into Duhem's Medieval Cosmology. I've included the parts which I felt lended themselves most to Ockham's view of multiple possible worlds.
2. Duhem was a world renowned philosopher and scientist and one of the few historians of science to praise the work of the Middle Ages. His book, Medieval Cosmology and The System of the World are both well known and oft cited.
3. Ockham had to fight against the Airstotlian mechanics which permeated his time and as such I felt the need to include a quote from Aristotle's On the Heavens.