Actually, a mind much greater than mine has already answered this question. In the Summa Theologiae, Saint Thomas Aquinas says: "This law is called natural because ..." and then he gives the answer. However, for our blogging purposes, let me try to put it in my own words. I have no book in front of me as I write:
The natural law is known by human reason, which is the very thing [reason] that makes our nature what it is. That's why it's called "the natural law." It is known through our nature, which is called "rational animal."
