The Unmoved Mover
The Cosmological Argument is an argument for the existence of a First Cause (or instead, an Uncaused cause) to the universe, and by extension is often used as an argument for the existence of an "unconditioned" or "supreme" being, usually then identified as God. It is traditionally known as an argument from universal causation, an argument from first cause, the causal argument or the argument from existence.
The basic premise is that something caused the Universe to exist, and this First Cause must be God. It has been used by various theologians and philosophers over the centuries, from the ancient Greek Plato and Aristotle to the medieval St. Thomas Aquinas and beyond. It is also applied by the Spiritist doctrine as the main argument for the existence of God.
The First Cause
A version of the cosmological argument could be stated as follows:
1. Every finite and contingent being has a cause.
2. A causal loop cannot exist.
3. A causal chain cannot be of infinite length.
4. Therefore, a First Cause (or something that is not an effect) must exist.
According to the argument, the existence of the Universe requires an explanation, and the creation of the Universe by a First Cause, generally assumed to be God, is that explanation.
In light of the Big Bang theory, a stylized version of argument has emerged (sometimes called the Kalam cosmological argument, the following form of which was set forth by William Lane Craig:
1. Whatever begins to exist has a cause.
2. The Universe began to exist.
3. Therefore, the Universe had a cause.
The idea that the universe has a cause is not often a point of contention here. We understand that something must have caused the change from Singularity to Big Bang. Nothing just is without a reason why it is. Everything that is has some adequate or sufficient reason why it is. This is the Principle of Sufficient Reason. Everything that comes into existence does so for a reason. The universe is a long chain of things that come into existence, with all of the links in the chain eventually connecting to the Universe itself, but does the universe as a whole have a cause? Is there a first cause, an uncaused cause, a transcendent cause of the whole chain of causes?
If not, then there is an infinite regress of causes, with no first link in the great cosmic chain, and the whole chain, no matter how long, would fall.
If so, then there is an eternal, necessary, independent, self-explanatory being with nothing above it, before it, or supporting it. It would have to explain itself as well as everything else, for if it needed something else as its explanation, its reason, its cause, then it would not be the first and uncaused cause.
Why must there be an uncaused cause? If there is no first cause, then the universe is like a railroad train moving without an engine. Each car's motion is explained proximately by the motion of the car in front of it: the caboose moves because the boxcar pulls it, the boxcar moves because the cattle car pulls it, et cetera. But there is no engine to pull the first car and the whole train. That would be impossible, of course. But that is what the universe is like if there is no first cause: impossible.
Peter Kreeft offers another analogy,
"Suppose I tell you there is a book that explains everything you want explained. You want that book very much. You ask me whether I have it. I say no, I have to get it from my wife. Does she have it? No, she has to get it from a neighbor. Does he have it? No, he has to get it from his teacher, who has to get it. . . et cetera, etcetera, ad infinitum. No one actually has the book. In that case, you will never get it. However long or short the chain of book borrowers may be, you will get the book only if someone actually has it and does not have to borrow it."
He goes on to explain,
"...existence is like that book. Existence is handed down the chain of causes, from cause to effect. If there is no first cause, no being who is eternal and self-sufficient, no being who has existence by his own nature and does not have to borrow it from someone else, then the gift of existence can never be passed down the chain to others, and no one will ever get it. But we did get it. We exist. We got the gift of existence from our causes, down the chain, and so did every actual being in the universe, from atoms to archangels. Therefore there must be a first cause of existence, a God."
In other words, dependent beings cannot cause themselves. They are dependent on their causes. If there is no independent being, then the whole chain of dependent beings is dependent on nothing and could not exist. But they do exist. Therefore there is an independent being.
Nature of the Cause
One objection to this argument, and probably the most obvious, is that while it does seem to show that there is a First Cause, it says nothing about that beings characteristics. This cause has an equal chance of being Jehovah as it does of being Zeus, Ra, Buddha, or the Pre-Big Bang Singularity. If there is such a necessary being, than that beings characteristics could conceivably be determined by other sources, but there is absolutely nothing in this argument regarding the nature of the First Cause, only that there must be one.
Anything You Can Do I Can Do Better.
Some object to the Cosmological Argument because any reasoning that can apply to God in this case may also be applied to the universe itself. For example, if it is admitted there must be a first uncaused cause of everything, why must that cause be God? Why can't it be the Singularity? Well, it can. As I've said, this argument makes no claim regarding what the cause was like, only that it must exist.
Who designed the designer?
It is sometimes argued (e.g., by Bertrand Russell) that there is a self-contradiction in the argument, for one of the premises is that everything needs a cause, but the conclusion is that there is something (God) which does not need a cause. This is actually based on a misunderstanding of the argument. It is never claimed that "everything" needs a cause, only that "everything which begins to exist" needs a cause. According to Christianity, there was never a point at which God began to exist. He has always existed and has no beginning. In other words, He needs no cause because he is not an effect.
Why can't there be an infinite regress?
Sometimes people wonder why there can't be an infinite chain of causes stretching off into eternity in both directions. Infinite regress is perfectly acceptable in mathematics: negative numbers go on to infinity just as positive numbers do. So why can't time be like the number series, with no highest number either negatively (no first in the past) or positively (no last in the future)? The answer is that real beings are not like numbers: they need causes, for the chain of real beings moves in one direction only, from past to future, and the future is caused by the past. Positive numbers are not caused by negative numbers.
However, there is, in fact, a parallel in the number series for a first cause: the number one. If there were no first positive integer, no unit one, there could be no subsequent addition of units. Two is two ones, three is three ones, and so on. If there were no first, there could be no second or third.
This argument fails in some ways, but succeeds in others. It does show that there must be some entity which simply always existed and has no cause. However, it does nothing to show that the Biblical God exists. Whether or not this Uncaused Cause is a personal, loving, eternal deity or an inanimate point of matter with infinite density and mass, but zero volume, depends entirely on the success or failure of other related arguments. One thing is sure though; whatever God is, it must exist.