Can the Zero Aggression principle point out to the direct way to act? No it’s so vague as to be useless.
Right-Libertarians, “Anarcho”-Capitalists, and assorted propertarians very frequently cite the Non-Aggression principle or Zero Aggression principle (Commonly called NAP or ZAP) as a core tenet of their ideology. It is brought up as the building block of voluntaryism on which free markets can be built and proudly displayed to show how morally superior such a society would be compared to anything else which, by the absence of the NAP, is defined to have an involuntary aspect.
But what exactly is the NAP? The specific details might differ depending on the encompassing ideology, but the central point generally seems to be that no human should aggress over another human. This is meant to mean the initial use of coercive force as well as the threat of such.
Now, if left to this end, this is not a half-bad principle, basically saying that people shouldn’t attack or threaten to attack others. However at this stage, it is also pretty much unnecessary to be given an explicit existence as a “principle” as the generic principle of freedom already encompasses this (i.e. attacking another person would violate their freedom). Other moral theories, particularly the utilitarian variants already encompass such rules (with stipulation) as a natural consequence of their suggestions. In the end, this basic form of non-aggression ends up sounding like a shallow “Thou shalt not kill” which, while pretty clear, when strictly adhered to can lead to worse results (such as foregoing killing in self-defence) or requires a more advanced moral framework above it which clarifies when it is, in fact, acceptable to kill.
But propertarians do not generally leave it at just that but rather try to sneakily expand it by linking it with private property rights. You see, the NAP is frequently derived directly from the Self-Ownership “axiom” and thus the wrongly derived property rights are treated as an extension of the self. Therefore one can then treat violation of private property rights as an act of “initated force”, even though no actual violence or threat of violence has been perpetuated. This is turn is used as a cause to use actual violence or threat of violence on the violator of property rights.
It thus becomes that the NAP, when combined with Self-Ownership conveniently becomes an excuse for someone to initiate real, literal violence against someone else. The right to freedom or utilitarian moral rules reserve the right for people to defend themselves against aggression, that is, to take only as much action as needed to stop the aggression against their person. This is pretty self-evident when achieved, both to the one being attacked and to any observers (i.e. it’s obvious when two people have stopped exchanging blows and threats). When extended to private property however, things get far far more complex.
While it’s easy to understand that someone “aggresses” when they steal something from another person (which is why most other moral systems do not require a NAP to label theft as wrong), things get pretty murky when one goes beyond that. Do I “initiate force” when I use a productive machine without paying rent? How about if I pay only enough rent to cover the cost of the machine? Do I “initiate force” when I toil the unused land that is owned by someone else? How about when I trespass?
This is further complicated by the claims of the NAP proponents that the NAP does not excuse any and all acts of self-defence but is rather limited by the level of aggression. We’re informed that it does not in fact, grant the right of shooting trespassers. But this again does not really clarify the matter. Whereas in literal aggression, one is always aware of the level the initiator is using (threats, shoving, punching, lethal weapons etc) and can respond in kind, in this extended field of aggression you’re left to comparing apples with oranges. What is the correct response to someone trespassing your property? Trespassing on their property? Forcibly taking them out? Threatening to shoot them and then follow through if they don’t comply? The truth of the matter is that unlike literal aggression, you cannot discover how you can respond in kind intuitively.
Thus we see that unlike actual aggression where equal reaction can cancel the aggression (i.e. shoving can defend against shoving, punching can defend against punches etc), in “aggression” on property via the NAP, the self-defence enacted is and must always be different and stronger than the act of “aggression”. A trespasser cannot be removed by counter-trespassing. They must be forcibly removed and this is very likely to require (threats of) lethal force if they do not comply. This get even more complicated if that person does not accept the NAP and considers the literal acts of aggression against them as the initiation of force it is and defends in kind.
There is no solution to this issue. The NAP, as a moral rule is incapable of making any suggestions or providing any solutions as it does not say anything substantial other than the vacuous “Thou shalt not initiate violence”. Rather we are told that an extensive legal system will be required which will either interpret some kind of “natural law” or be somehow employ objective judges which make the perfect decisions. Something like the system of wealthy libertarian judges that Murray Rothbard proposed, which would follow some kind of “libertarian law”. In short, nothing more than a subjective legal system built around the principles that people like Rothbard prefer.
And all these issues occur before we even consider that extrapolating private property rights from the “axiom” of Self-Ownership is a non sequitur as it’s impossible to deride a particular set of ownership rights out of it (which is why you can see how much and how many libertarians disagree on what specific ownership rights to use). Due to this, a NAP that ideologically protects a particular set of ownership rights is nothing more than a subjective argument against the things a particular person does not like. That one does not like people trespassing on his property so he calls that “an initiation of force”. That other one does not consider trespassing to be such, but it’s certainly an “initiation of force” when workers don’t pay him rent for a factory’s costs he’s already recovered.
The NAP is shown to be pretty much a shallow principle. When limited to actual physical force, it’s superseded and made obsolete by moral systems which can explain when force is justified or not. When extended to concepts which are not immediately intuitive, its subjective nature quickly devolves it to shouting matches which can only be settled by a homogeneous system of courts and enforcement agencies. A de-facto state.
To me, when someone explains that according to the NAP, this or that is wrong, they mostly sound like “This or that is wrong, because I say so.”