I want you to imagine that a Martian came to Earth and was told what a Unicorn is, what a Narwhal is and what a Triceratops is. The Martian, from these descriptions would be unable to determine which of these things currently exists, once existed or has never existed. All three have horns. All three are strange. All three are, strictly speaking, possible.
This points out a basic truth of the things that make up our physical world: Their essence or their “what-ness” is different from their existence. That is to say, knowing what something is does not bring you an inch closer to knowing whether that thing has been instantiated. Some things are but their reality is not necessary.
We have a world where Triceratops are no more and there has never been a unicorn…but under a different set of causes a different state of affairs could have been.
When something can be but does not have to be we look for an explanation for why it exists. All contingent beings, that is to say all things that can be but don’t have to be, are a composite of essence and existence; these two things do not imply each other and must be brought together by outside causes. That is to say, if something does not need to exist, it cannot be the cause of its own existence.
However if we appeal to contingent causes to explain contingent realities those causes will need to be explained, and those explained and those explained. This cannot go on forever without violating the principle of sufficient reason.
So we arrive at the realization there must be a reality whose what-ness does imply its existence, a reality that is ipsum esse subsistents the subsistent act of to-be itself. This, as Aquinas would say, all men call God.
To be the sheer act of to-be itself, this reality would have to reject all forms of complexity, because otherwise it would start to take on contingent properties (if its something other than the act of being, it will need an explanation of how it came to be). This is why there can’t be multiple gods. To have multiple gods there would have to be something that differentiated between them, and that would imply a complexity.
This is just a very brief introduction to the argument from contingency. I hope to explore more on this blog.