What is "agency," anyway?

I've been trying to wrap my mind around agency.

I understand the standard Mormon theological definition - agency refers to our ability to make decisions that can either damn us or exalt us.  These decisions are naturally made from within the framework of our environment.  But if you trim away all the external circumstances, all the environmental influences, there is a little gem called "agency" that is supposedly the prime motivator of all human behavior.  Agency is what allows us to "act," and not be "acted upon."

Unfortunately, my little brain has a hard time comprehending what that actually means.  The engineer in me wonders what the mechanism is that causes us to exercise our agency in one way as opposed to another.  But if agency is really free will, uninfluenced by outside circumstance, then it has no "cause."  It just IS.  Agency can't be explained in terms of anything we know because doing so would reduce it to an effect.  The only way to define agency is therefore in terms of itself - agency is agency.  Period.

In a lot of ways, it seems like agency is Aristotle's "Prime Mover," the one thing that can't be explained in terms of anything else.  Every chain of cause and effect goes back and ultimately stops at agency.  Why is the sky blue?  Agency.  Why does the earth exist?  Agency.  Where does God come from?  Agency.

It seems like agency is this permanent, pre-existing, omnipresent force, even more permanent and eternal than God Himself.  Or maybe God IS agency.  Whatever.