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The Cosmological Argument for the existence of God, as propounded by Thomas Aquinas, is also known as the Third Way. It is the Third of Five ways in Aquinas's masterpiece, "The Summa" (The Five Ways).
The Cosmological argument is developed around a distinction between that which has necessary existence and that which is contingent. A thing that has necessary existence must exist in all possible worlds, whereas a thing that is contingent may go out of existence.
The method Aquinas uses is to set up the contrary position, then prove it to be wrong. Therefore, the cosmological argument begins by accepting the premise that all things are contingent. If all things are contingent, i.e., if all things can go out of existence and do not necessarily exist, then there must be a time where all things go out of existence.
Aquinas appeals to the *Principle of Plentitude* at this juncture, which states that if something is a real possibility, then given an infinite amount of time, it should happen. Real possibilities show up. It is a real possibility that if everything is contingent, everything could go out of existence at once, given that time is infinite at any point, such as now.
If this is the case, then there would be nothing now--but such an idea is absurd since we have the evidence of existent things which we can perceive. Yet, that could be because everything pops out of existence, then back into existence. Aquinas answers this from the princilple of *ex nihilo, nihil fit*--if something pops out of existence, it cannot pop back into existence. Once something ceases to exist, it cannot suddenly exist again. Our sense perceptions and experience tell us that something exists, something is there, therefore everything has not ever gone out of existence. Therefore, all things cannot be contingent. Corollary to this is the conclusion that there must then be such a thing as a necessary existence.
Aquinas furthers his premise by supposing that there is a hierarchy of being with necessary existence, each lower being dependent upon the higher to infinity. The hierarchy of necessary existence itself would need an explanation for its existence. Here, Aquinas appeals to the *principle of sufficient reason*, which states that everything that happens has to have a sufficient explanation for occurences. Since the hierarchy of necessary existences would therefore need to be explained, because of the principle of sufficient reason, it would need a self-explaining necessary being, standing outside the series, to explain the order of the hierarchy. Deductively, that self-explaining necessary being would be that which humans understand to be God. Hence, the definition of "God" that Aquinas arrives at is this: The self-explaining necessary existence upon which all things are contingent in all possible worlds.
David Hume provided a rational path out of the Cosmological argument for the existence of God according to Aquinas. Writing during the Scottish enlightment of the 1700s, Hume claimed that the principle of "ex nihilo, nihil fit" is untrue; if something can pop into existence and pop out of existence, then it cannot also pop back into existence again. Thus, according to Hume, all things may be contingent, without any need for necessary existence.
The Third Way of Aquinas argues logically for the existence of a god, but doesn't necessitate the Judeao-Christian God of his own tradition. Aquinas tried to fill this gap by claiming that a being of self-explaining, necessary existence would by nature possess the attributes which are suggested in nature to the point of perfection. Thus, since human beings are imperfect personalities who reason, emote, act, communicate, etc., God also possesses analogous qualities in perfection. Also, Aquinas argued, that which is found in the ultimate cause must also be found in the offspring, a line of reasoning that led to the Fifth Way, the argument from Design.
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