Whereas contract law is whatever the contracting parties may agree to so long as it is not inconsistent with natural law (emphasis mine), natural law principles and rules and the procedures for protecting and enforcing them are not as easily identified.
Which speaks to me of the Bubble Theory of Property Rights. In fact, this is the same argument, probably worded better, that I have made in the past. A contract which violates natural law is null and void on those points before it even has a chance to be enforced. Self defense and your personal property, which surrounds and infuses your physical body wherever you may be, are natural laws that can't be nullified by contract.
Or, at least that is how I see it.