Aristotle's Cosmological Argument
The mathematical science that is closest to philosophy, i.e. astronomy. For astronomy studies a kind of substance that is perceptible, but still everlasting, whereas the other mathematical sciences – those concerned with numbers and with geometry, for instance – do not study any substance at all. Aristotle
15th Century Latin Translation of Metaphysics
University of Oklahoma: History of Science Collections
Weblink: Metaphysics: Book One
Aristotle’s works span a wide range of topics: from logic to rhetoric, ethics to politics, physics to biology. However his work that has perhaps had the greatest impact on religion and theism in general are the fourteen books spanning nearly 114 pages which make up Metaphysics. In book XII of Metaphysics Aristotle puts forth the first cosmological argument (an argument derived from the study of the universe) for the existence of god. The basic cosmological argument, which would later be further developed by Aquinas in the thirteenth century and Leibniz and Clarke in the eighteenth century, follows:
- Every being (that exists or ever did exist) is either a dependent being or a self-existent being.
- Not every being can be a dependent being.
- Therefore, there exists a self-existent being. 
According to Aristotle motion can neither come in to existence nor cease to be for it has always been, with the same logic being applied to time since there can be no before or after if there is no time.  “There must then, be a principle of the sort whose essence is actuality.”  (In contrast with something which has the essence of potentiality).
Here begins Aristotle’s idea that there must be some first cause for all matter, referred to in the philosophy of religion as the First Cause Argument:
For how will things be moved if there is no cause in actuality? For surely matter will not initiate motion in itself, but carpentry [for instance, must initiate the motion]; nor will the menstrual fluid or the earth initiate motion in themselves, but the semen and the seeds [must initiate the motion]. 
He goes on to present what many objectors of his era may use as a weapon against his beliefs, that being the possibility of the eternality of the universe:
Hence some people – Leucippus and Plato, for instance – believe in everlasting actuality; for they say that there is always motion. But they do not say why there is this motion, or what kind of motion it is, and neither do they state the cause of something’s being moved in this way or that. For nothing is moved at random, but in every case there must be some [cause] – as in fact things are moved in one way by nature and in another by force or by the agency of mind [understanding] or something else. 
Hereafter the idea of the first cause becomes rephrased into that of the unmoved mover, attributing the mover with eternal existence in a physically representative body:
There is something, then, that is always being moved in a ceaseless motion, and this motion is circular (this is clear not only from argument but also from what actually happens); and so the first heaven is everlasting. Hence there is also something that initiates motion. And since whatever both is moved and initiates motion is an intermediary, there is something that initiates motion without being moved, something that is everlasting and a substance and actuality. 
He goes further in his analysis of the unmoved mover in describing the activity of his existence in comparison to our own:
For the primary mover is always in this state [of complete actuality], whereas we cannot always be in it; for its actuality is also pleasure (that is why being awake, perceiving, and thinking are pleasantest, while expectations and memories are pleasant because of these). 
Finally, the unmoved mover must be whole and unchanging either by itself or by other substance:
…there is an everlasting, unmoved substance that is separated from perceptible things. It has also been proved that this substance cannot have any magnitude, but must also be without parts and indivisible; for it initiates motion for an infinite time, but nothing finite has infinite potentiality. And since every magnitude is either infinite or finite, [the primary mover] cannot have a finite magnitude, and it cannot have an infinite magnitude, because there is no infinite magnitude at all. 
From the idea that every substance has a cause Aristotle goes further, and with the use of his works in Physics and On the Heavens, begins the analysis of the motions of the planets and stars:
…besides the simple local motion of the whole, which we say is initiated by the primary and unmoved substance, there are also the everlasting local motions of the planets [the revolutions around the Earth]. It necessarily follows that each of these motions is also initiated by the agency of some substance that is unmoved in its own right and everlasting. 
From this idea it is inherent that there are more motions than there are bodies of the universe because according to Aristotelian physics the way an astronomical body behaves is described by multiple forces being performed on it at once:
For if everything that initiates local motion is for the sake of what is moved, and if every motion is the motion of something that is moved, then no motion is for its own sake or for the sake of some other motion, but every motion is for the sake of the stars. 
Here we have the key points of Aristotle’s argument for the existence of god. Whether that god is a theistic god is quite another debate and while it is unlikely that he intended this use for his formulation, his unmoved mover is an excellent starting point for any theistic or philosophical debate in religion. We conclude with a slimmed down and simplified summary of Aristotle’s argument:
- Motion and time cannot appear or disappear
- Things cannot move without a cause.
- There must be something that causes the initial movement of the universe.
- That something is eternal and existing, always.
- It is without parts and indivisible, and unable to be perceived by us.
- It is the cause of the movement in the universe and is responsible for those forces which cause subsequent movements in the planets and the stars.
Aristotle finishes the section of the unmoved mover and leads us in to the next episode on the plurality of worlds with the following:
It is evident that there is only one heaven [Aristotle used heaven as both contrasting the earth and including the earth in different materials]. For if there were a number of heavens, as there are a number of men, then the principle [unmoved mover] – one for each heaven – would be specifically one but numerically many. Now, things that are numerically many all have matter; for many [particulars] have one and the same account – that of man, for instance – but Socrates is one [particular]. But the primary essence [unmoved mover] has no matter, since it is actuality. Hence the primary and unmoved mover is one in number and account; so also, then, is what is always and continuously moved; and so there is only one heaven. 
 Rowe, William. Philosophy of Religion. 4th Edition. 2007
 Aristotle. Metaphysics. 1071b7
 Aristotle. Metaphysics. 1071b7
1. Aristotle's Metaphysics is a hefty read to say the least. The Metaphysics is the cornerstone of this work, especially given it is the main source for Aristotle's theistic cosmology, and as such I have relied on it heavily. There are many work's of Aristotle's which are of dubious origin, such as On the Universe and Mechanics, however Metaphysics is not one of them and is believed by most to have been compiled from his own lecture notes. Note: The citation numbers are those of the Bekker system developed for the arangement of Aristotle's works in the 19th century.
2. William Rowe is not a historian of science per se, though I'm sure he'd have a great deal more knowledge on its history than most. His book on the philosophy of religion is a staple to most introductory classes on the subject. He is a professor emeritus of philosophy at Purdue University and the Dictionary of Modern American Philosophers has said that he has helped cause a "remarkable revival of analytic philosophy of religion since the 1970s."