FIRST LAW: The Significance of the Title

[Nathan]

"It is forbidden to touch The Other Side direct. Forbidden to communicate with the world below, forbidden to summon Demons, forbidden to open gates to hell." -- Bayaz, Before They are Hanged

I think Abercrombie named his series The First Law as allusion to the personal demons his characters try, albeit ineffectually, to suppress, the best example being (of course) The Bloody Nine. I’m interested in what you guys think. Why is the series called The First Law?