1. In the world, there are contingent things, things that can either be or fail to be.
2. If everything can fail to be, then at some time there was nothing in existence.
3. If at one time there was nothing in existence, it would have been impossible for anything to have com into existence. (You can not get something from nothing)
4. Therefore, not all beings are contingent--merely possible, but there must be something that exists necessarily ad of itself.