I think the non-aggression principle is true, but sort of useless in an argument with a non-libertarian. It’s a conclusion, not a premise. Similarly, I think it’s true that we’re self-owners, but I think self-ownership is close to useless in an argument with a non-libertarian. It’s a conclusion, not a premise. Let me explain why.

For the sake of argument, suppose that the Bible is literally true. Now, consider the following argument:

1. If the Bible says X, then X.
2. The Bible says there is one God, who created the universe in six days.
3. Therefore, there is one God, who created the universe in six days.

The argument above is valid. It is impossible for the premises to be true and the conclusion to be false. Now, on the supposition that the Bible is literally true, it would follow that the argument above is also sound. It would be a valid argument with true premises. But, even still, this would be a lousy argument for a Christian to use against an atheist. Imagine how that might go:

Christian: “Do you believe in God?”
Atheist: “Hell, no!”
Christian: “Ah, well I have a knock-down argument against atheism. Here goes: [insert the argument above here]”
Former Atheist: “That argument is airtight. I now accept your religion.”

The problem is that even if the argument were sound, it begs the question against the atheist. It assumes something that the atheist disputes with the Christian. Good arguments do not do that. A good argument is in that sense relative to the target of the argument. With whom are you arguing? A good argument starts with common ground and commonly accepted principles. It does not presuppose something that the person you’re arguing against disputes, especially something that is more or less the very thing being argued about. (In my forthcoming book Why Not Capitalism?, I basically grant G. A. Cohen all of his premises about how to do political philosophy and pretty much all of his premises about morality, but I then argue that what follows is that anarchist-capitalism, not socialism, is just. A perfectly just society would be anarcho-capitalist. I agree with Cohen that Rawls’s A Theory of Justice isn’t actually a theory of justice at all.)

With that in mind, this is why the “non-aggression principle” doesn’t do the work many libertarians hope it does. The problem is that libertarians who use the NAP often presuppose the very things their interlocutors dispute. So, for instance, suppose an unsophisticated libertarian were arguing with a sophisticated Marxist. The argument might go like this:

Libertarian: “Are you a libertarian?”
Marxist: “Hell, no!”
Libertarian: “Do you think it’s acceptable to initiate force or aggression against another person or to violate that person’s rights?”
Marxist: “Of course not! That’s why I’m a Marxist and why I reject libertarianism. After all, everyone has an extensive set of rights over himself. What’s more, capitalism is wrong because the capitalist illegitimately appropriates what should be common property to himself, and then uses his illegitimate claim to that property to seize the laborer’s labor from the laborer. This violates the laborer’s self-ownership.”

Now, not all Marxists talk that way. But some do, and many more could. The point here is that both the libertarian and Marxist can agree that it’s wrong to initiate aggression, but they dispute who has a claim to what and so what counts as aggression. Neither one of them can resolve the dispute by pointing to the wrongness of aggression. Rather, they need an independent argument determining who owns what. You don’t reason your way from non-aggression to libertarian theories of property rights. Rather, you derive the libertarian non-aggression principle from a libertarian theory of property rights.

Similarly, imagine an argument between a libertarian and a left-liberal Rawlsian:

Libertarian: “Are you a libertarian?”
Rawlsian: “Hell, no!”
Libertarian: “Do you think it’s acceptable to initiate force or aggression against another person or to violate that person’s rights?”
Rawlsian: “No.”
Libertarian: “Then how can you advocate social democracy, government regulation of the economy, and high taxes?”
Rawlsian: “These don’t violate people’s rights. Property rights in anything more than our basic personal property are legitimate only if part of a scheme that tends to benefit all people subject to that scheme, and so taxes are justified as a means to ensuring the legitimacy of our property. Similar remarks apply to regulation of the economy. You have a right to follow whatever religion you want. Even  if it turned out that your choosing to be Catholic better helped the poor than your choosing to be Methodist, you could still be Methodist. That’s within your rights. It’s similarly within your rights to refuse to date black people, even though that’s racist. But you have no rights, grounded in justice, to open up a store that refuses to hire black people, or that is free of regulation. Instead, the amount of economic liberty you get is determined by what system turns out to best produce the difference principle and fair equality of opportunity. In fact, I oppose libertarianism because it violates people’s rights.”

Smart Rawlsians and smart Marxists do not disagree with conventional libertarians that aggression is wrong. Rather, they dispute what our rights are and who owns what. They thus dispute what counts as aggression. The libertarian says, “Taxes are theft!” The Rawlsian says, “Failing to pay your taxes (in a proper Rawlsian state) is theft!” Who is right depends not on the NAP, but on more fundamental questions of who owns what and what our rights are.

If libertarianism is true, it follows that Rawlsian societies do indeed violate people’s rights wrongfully, and wrongfully aggress against innocent. However, the Rawlsians do not describe themselves this way. Rawlsians don’t say, “Oh, sure, the federal government taxing Jason Brennan at 33% violates his rights, but it’s worth it so we can feed greedy corporations and subsidize student loans.” Rather, they think I don’t have any claim to the income they take away. They think that money belongs to the government, not me.

The reason certain arguments seem like knock-down arguments to libertarians is not that the arguments are good, but that they beg all the right questions.