The Leibnizian Cosmological Argument argues for the existence of God based off of the contingency of the universe’s existence. In other words, since the universe exists and everything that exists has an explanation of its existence, then God must exist.
The Leibnizian Cosmological Argument, or Contingency Argument can be formulated as follows:
1. Anything that exists has an explanation of its existence either in the necessity of its own nature or in an external cause.
2. If the universe has an explanation of its existence, that explanation is God.
3. The universe exists.
4. Therefore, the universe has an explanation of its existence.
5. Therefore, the explanation of the existence of the universe is God. (Reasonable Faith, 106)
Premise 3 is not controversial at all. I mean, if the universe didn’t exist, then we wouldn’t be here discussing it. What is debated is whether the universe has an explanation for its existence whether that explanation is God.
Everything has an explanation if its existence whether it exists necessarily or contingently. Things that exist contingently exist because something caused their existence. You and I exist because our parents met. A watch exists because it was made by a watchmaker. Things that exist necessarily necessarily exist because of their nature (e.g., God).
Some will question why the universe doesn’t exist necessarily just like God. The universe is physical like you and I. Why would it exist necessarily? We can imagine a time when objects we observe did not exist. When the universe was very dense and hot, nothing we observe existed. Craig brings up the example of the fundamental particles or building blocks of matter, such as quarks.
Well, it’s easy to conceive of a world in which all of the fundamental particles composing some macroscopic object were replaced by other quarks. A universe consisting of a totally different collection of quarks, say, seems quite possible. But if that’s the case, then the universe does not exist by a necessity of its own nature. For a universe composed of a wholly different collection of quarks is not the same universe as ours. … If it were composed of a different collection of quarks, then it would be a different universe, not the same universe (Reasonable Faith, 109).
We know from experience and observation that everything has a cause for its existence. We can extrapolate this to the universe, a physical object bound by the same natural laws that everything else is. The size of it does not change anything. Therefore, the universe exists contingently and has a cause.
Others will inquire what caused God. If God created the universe, all of space-time matter, then he must exist outside and differently than that of space-time matter. He is not bound by the same laws of nature.
The universe exists contingently. Therefore, it has an explanation for its existence, and that explanation is God, a necessarily existing being.