This paper contends that the requirement of content independence poses a pressing challenge to natural-duty theories of political obligation, for it is unclear why subjects of a state should not discharge the background natural duty in proper ways other than obeying the law. To demonstrate the force of this challenge, I examine and refute three argumentative strategies to achieve content independence represented in recent notable natural-duty theories: by appealing to the epistemic advantages of the state in discharging a natural duty, (...) by claiming that one’s denial of state authority necessarily poses an unjust threat to other people, and by invoking the consideration of fairness to preempt one’s discretion in discharging a natural duty. My criticisms, I believe, provide sufficient reasons for natural-duty theorists to take the requirement of content independence much more seriously. (shrink)