How about the Gospel? There's not much savor of medieval natural law to be found in the words of Christ. And while you might respond, well, there's not much "system" there either, and for that we look to the Church, I would respectfully disagree. Christ purposefully downplayed, and often challenged the systematic and rule-driven (of which there was plenty in his time) in order to place the emphasis, instead, on first principles, such as love and mercy. A system without much system. And a radical advance in moral thought.