An argument for the
existence of God.
If you look at history--the chain of cause and effect through time--it's obvious that the chain has to have a starting point. In its crudest form, the first cause argument says, "the universe had to come from somewhere, and that somewhere is God."
But why doesn't God have to come from somewhere? Many modern wielders of the first cause argument can't answer that coherently, but the answer dates back to Aristotle. He postulated the first cause as a sort of metaphysical magnet. The first cause initiates change not by acting, but by being the focus and goal--the telos--of the actions of everything else. The unmoved mover doesn't need a cause to explain its changes, because it doesn't actually change...it remains totally stable, while everything else changes around it.
There are a couple of consequences to this. First, if you're a Greek philosopher, you're going to figure that everything will be naturally attracted toward good and fullness of being. Thus, the first cause must be the ultimate being and the ultimate good--in other words, God. Second, you have to figure out why everything moved away from the prime mover in the first place; this provides an impetus for all sorts of cosmological and dualistic theories about falls from grace.