 Other advocates of the view that there is a distinction between negative and positive rights argue that the presence of a police force or army is not due to any positive right to these services that citizens claim, but rather because they are natural monopolies or public goods—features of any human society that arise naturally, even while adhering to the concept of negative rights only.Robert Nozick discusses this idea at length in his book "Anarchy, State, and Utopia".In the field of medicine, positive rights of patients often conflict with negative rights of physicians.