Jason Brennan provides an excellent explanation why some libertarians are mistaken when using the non-aggression principle (NAP) as an argument against non-libertarian ethical theories. This is an important point and it is worth repeating to those who tend to forget it. However, we need to be careful when disqualifying the NAP as a justification for libertarianism not to disqualify its proper place within the libertarian theory. That special place refers to the operational properties of the NAP within the libertarian theory.
While it is true, as Brennan demonstrates, that the NAP could be incorporated into other theories of property rights, this principle is fully operational only within the libertarian theory. Why? Because, unlike in the utilitarian, pragmatic, or legal positivist theories of rights, where rights are defined as bundles of permissible actions, rights within the utilitarian theory are defined as an absolute jurisdiction over a certain area of space or over an object for a specified period of time. If we take as given that a person, say Jim, owns a house and a plot of land around that house, within the libertarian theory of rights, this ownership is defined as an absolute jurisdiction over the space limited by the boundaries of Jim's plot of land (we may define this space to a certain height as well). As long as Jim uses the property within this defined space in a way that no physical objects interfere with someone else's (say, Janis's) space (we are assuming that both Jim and Janis are rightful owners of their properties), Jim is operating within his property rights.
The NAP is an operational concept in the sense that it is a label we put on a situation in which Jim would place a physical object into the space under Janis's jurisdiction without Janis's consent. This label says that Jim's action has violated Janis's absolute jurisdiction over her property. Thus, Jim's action is unjust, and we call it aggression. Jim should not aggress because this action violates the principle of Janis's absolute jurisdiction over a defined area. Again, remember we are assuming we have already solved the problem of justifying Jim's and Janis's ownership over their respective areas of space. Their just ownership is used as a basis for labeling certain physical interferences as aggression.
The NAP could be applied outside of the libertarian theory of rights. So, we could use some other theory to justify ownership and then label certain actions as aggression. For example, we might use the utilitarian theory to justify taxation, and then label tax evasion as aggression against tax recipients, who would be the rightful owners of those taxes. However, while the NAP could be applied outside of the libertarian theory, it cannot be consistently applied and be operational. The reason for this is the way in which rights are defined within non-libertarian theories of rights--they are defined as bundles of permissible actions.
We can illustrate this with an example. This example is from an area of law where the distinction between the libertarian and non-libertarian theories of rights is particularly illuminating--environmental law.
Say Jim is a farmer, and Janis is the Minister of the Environment. Janis has the authority to determine which actions Jim has the right to perform and which actions he does not have the right to perform. For example Janis may demand that Jim applies fertilizer between April 1st and April 15th, or she may demand that Jim applies fertilizer by injecting it at a certain depth under the soil surface, or she may demand that Jim does not apply fertilizer during rainfall or snowmelt. We are assuming that Janis is fully justified in making these demands.
What happens if Jim does not obey Janis's demands. Since Janis is fully justified in expecting her demands to be fulfilled, Jim has committed an unjust act. But, can we say that Jim has committed an act of aggression against Janis's property? Although Janis has the right to determine how Jim is going to use his tractor, fertilizer and land, Janis does not own those objects. She has some jurisdiction over them, but not ownership in the sense in which the libertarian theory defines ownership.
Jim might have disobeyed the orders made by Janis, but it would be meaningless to say that Jim has committed an aggression against Janis's orders or demands. What we know is that he has not committed aggression against Janis's property, and we cannot use the NAP to label Jim's action as unjust. Although we cannot use the NAP to label Jim's action as unjust, we can still say it is unjust because Jim has disobeyed his moral obligations to Janis.
I used this example to illustrate the proper operational place of the NAP within the libertarian theory, and how this operational place cannot be always replicated in other theories of rights. In fact, my experience in environmental law indicates that instances in which this operational place of the NAP could be replicated within non-libertarian theories is quite small relative to those in which it cannot.