Well, Aristotle did it better than I ever could (in "The Metaphysics", which isn't nearly as kooky as it sounds; it just means it's the second volume of "The Physics").

What I do really love about it (and this plays out in medieval European and Islamic thought, both of which were strongly influenced by Aristotle) is that he says that for a living thing, the formal cause and final cause are the same thing. The various motions and activities of an organism (this is all Aristotle meant by "soul") are its final cause; my living my life as well as I can is my purpose.

That applied to the whole cosmos, too. Aristotle considered the cosmos alive (after all, the stars and planets move without anybody pushing them), which meant the purpose of the cosmos was to be the way it is and spin those crystal spheres and epicycles they imagined the planets moved on. And that final cause, that universe's-motion-as-its-own-purpose, he called "God" -- the omega point that the universe moves towards. In the renaissance and enlightenment this got reversed, and the "first cause" came to mean first in sequence rather than importance; from that you get Newton's "watchmaker" God, which would have been incomprehensible to a medieval thinker (let alone Aristotle).