Argumentation Ethics (AE) is an argument attempting to establish the objectivity of libertarian ethics originally put forward by Hans Herman-Hoppe. This is a hard thing to do because of the is-ought problem. Rothbard said of Hoppe and AE, "he has managed to transcend the famous is/ought, fact/value dichotomy that has plagued philosophy since the days of the Scholastics". That would be impressive if it were true.
Let's look at Hoppe's own and original formulation, from "The Ultimate Justification of the Private Property Ethic", in the September 1988 edition of Liberty. The contents can also be found at Mises.org in an article titled "A Primer on Hoppe's Argumentation Ethics". I have done what I can to shorten it without misrepresenting it or making it less clear. My extensive use of ellipses are to streamline the sentence when out of context, I can see how this may look like changing the meaning, so if you suspect foul play, please refer back to the preceding link to see it in its original context.
I demonstrate that only the libertarian private property ethic can be justified argumentatively...
Such a proposal [of a non-libertarian ethic] can be made, of course, but its propositional content would contradict the ethic for which one demonstrated a preference...by the act of engaging in argumentation as such...
To reach this conclusion and to properly understand its importance and logical force, two insights are essential.
First...the question of what is just or unjust...only arises insofar as I am, and others are, capable...of argumentation.
...any...proposition must be assumed to claim that it is capable of being validated by propositional or argumentative means. ... In fact, in producing any proposition, overtly or as an internal thought, one demonstrates one’s preference for the willingness to rely on argumentative means in convincing oneself or others of something. There is then, trivially enough, no way of justifying anything unless it is a justification by means of propositional exchanges and arguments.
...it must be considered the ultimate defeat for an ethical proposal if one can demonstrate that its content is logically incompatible with the proponent’s claim that its validity be ascertainable by argumentative means.
Second...argumentation does not consist of free-floating propositions but is a form of action requiring the employment of scarce means
...no one could possibly propose anything...if a person’s right to make exclusive use of his physical body were not already presupposed.
Anyone disputing such a right would become caught up in a practical contradiction since arguing so would already imply acceptance of the very norm which he was disputing.
His conclusion is that anyone disputing a person's right to make exclusive use of his physical body is making a proposal with content that is logically incompatible with implication that its validity is ascertainable by argumentative means, when using argumentative means requires property. In shorter terms, you have to use property to deny property, a performative contradiction. He goes on to try to rationalize homesteading as the only way to "sustain argumentation for any length of time and rely on the propositional force of one’s arguments".
...if no one had the right to control anything at all except his own body, then we would all cease to exist and the problem of justifying norms simply would not exist.
He pre-emptively responds to those who might claim an is-ought violation by expressing his reasoning toward the end:
(a) justification is propositional justification — a priori true is-statement
(b) argumentation presupposes property in one’s body and the homesteading principle — a priori true is-statement
(c) then, no deviation from this ethic can be argumentatively justified — a priori true is-statement
First, I'm going to skip the arguments that I've seen many others make elsewhere if I have nothing to add. You can read what others have said in the November 1988 issue, archived in PDF here. Second, I'd be happy if Hoppe had an effective argument. This would be useful to me. His intent is to show that only a private property libertarian ethic can be argued for because arguing for any other ethic would create contradiction, but that means the same thing as saying that intends to show that every other ethic than the private property libertarian ethic can't be argued for because arguing for any other ethic would create contradiction. If a good case was made here, I'd be able to argue it compellingly to people who believe non-libertarian ethics to unseat them from their current beliefs, and then push that refutation one ethic further than Hoppe does and bring them into my position, moral nihilism. Moral nihilism is not in the set of ethical positions that AE is capable of refuting, and Hoppe doesn't appear to try to refute it. Hoppe has nothing to say in AE to moral nihilists, he only tells every other brand of moral realist he thinks is a potential competitor in the minds of his audience that they are wrong.
Why isn't moral nihilism refutable by his argument? Because a moral nihilist proposes exactly zero norms, the highest possible number of norms that a nihilist can performatively contradict themselves on is zero, meaning they cannot performatively contradict their own norms.
The closest Hoppe gets to addressing this issue is in saying that in forming propositions to exchange in argumentation, one demonstrates a preference for relying on argumentation for persuasion. This "demonstrated preference" does not imply that the preference demonstrated is the highest preference, nor that it is an unconditional preference. It's amazing that such a simple insight goes unanswered because it's a clear aspect of our subjective experience of value. It would not be a performative contradiction for one who believes that argumentation is only sometimes preferable to argue their case that argumentation is only sometimes preferable, especially if they do so only sometimes. Similarly, choosing to use argumentation as a means to "justify" or establish the truth of a claim does not imply that no other mechanism would work, or that argumentation is an unconditionally preferred mechanism, and this is my position. I use argumentation as a tool to use sometimes, and while it is a tool that is useful very generally, it is not a tool that is always the most useful. But do I hold some general normative preference for it? Not as a rule, certainly, and any "demonstrated" preference is demonstrated as at most a tendency rather than a norm. And this isn't a position that only a nihilist can take, other moral realists can easily take the same position.
He also says a number of completely absurd things. The statement that no one could propose anything without a right being presupposed, for example, would be right if you assume that "could" means "morally allowed" rather than actual ability, but is clearly absurd as written. It seems other critics were less charitable in taking him literally, and he could certainly have made that line clearer. It's harder to interpret the idea that if no one could (morally) control anything but their own body we would "cease to exist" in a way that squares up with his other ideas, he's making the is-ought fallacy of using "our continued existence is good" as a hidden premise, and he seems be doing this using consequentialist reasoning which he specifically rejects. After all, it's conceivable that our continued existence is actually bad (non-existence means not suffering, for example) or that existing and not existing are both equally good or bad. Without giving a purely descriptive justification for this value statement, we have to assume his reasoning is now contaminated with the premise "we ought to continue existing", placing it clearly outside of purely descriptive rational ethics. Even if we were to grant everything leading up to this line (which I don't do), the argument Hoppe wants to make, that supporting private property, falls apart.
There is certainly more to say about things that weaken the argument without directly refuting it. For example, "self ownership" is such a weird kind of "property" that it doesn't seem to merit calling it property at all (e.g. it cannot be bought or sold, except in the view of "unironically" pro-slavery "libertarians" that I'll charitably ignore), and simpler concepts such as "autonomy" can be substituted without all the unnecessary baggage and subsequent proof of the unnecessary claims, and it's especially weird that the idea of property through "self ownership" was introduced before any of the things that people who aren't already libertarians understand to be "ownership". (It actually makes total sense once you figure out that libertarians don't care about liberty per se, only those liberties which can be expressed as property, and they are attempting to kidnap the term "liberty" to label an idea better described as propertarianism.) It's clear to any non-libertarian that words are being redefined in special ways, and probably (actually) in misleading ways. If Hoppe wanted to make a stronger argument, he could have chosen less loaded wording and words that he might have been able to more easily redefine for the purposes of his argument.
There's the aspect that this argument Hoppe makes is a revision of an older argument, AE is a variation on "discourse ethics" as proposed by one of Hoppe's teachers, but that argument was used to support social democracy, and it's very easy to see how this can be done. Instead of assuming for no particularly compelling reason that property is how you would express what people need in order to argue, substitute your favorite basket of basic needs (food, shelter, clothing, etc), and you can make an even stronger case that having the direct use of the things you need to argue is more directly related to your ability to continue arguing than having a only the ability to make a mere claim to ownership of only that subset of those things you need to argue that you've been able to command. I suspect that Hoppe's particularly egregious is-ought failure mentioned above correlates directly to a critical part of an argument you'd find in discourse ethics, as if you didn't have the basic needs for argumentation, you do much more directly run into problems of continued existence than if you merely cannot own them. It wouldn't be much of a stretch at all to conclude that communism is a presupposition of argumentation or discourse on ethics.
There's the problem of all of his failures to exhaustively refute alternatives, which would most easily be done by having categories that can be shown to be properly exhaustive, when trying to demonstrate the "necessity" of a particular stance. The best examples of this surround his demand that property be defined in exclusively physical terms. He assumes that, if everyone owned everything regardless of physical relationships to these things, you'd need the permission of every single other person to do anything, similar to how he assume that property can only take the form of a right of exclusive control. What if property, rather than a "right" to control, were conceived simply as the person to whom a debt is owed for the (necessarily) subjective damages done, and that nobody was "bound" in an way to obey an owner of property except insofar as they might owe a debt? You wouldn't need everyone's permission in that case, only reason to think there would be no debt created by your use of something, or an acceptable debt. The problem of getting everyone's consent is the biggest part of the reason why he doesn't think communism could be supported by this idea (which nevermind everything else is a strawman). You also wouldn't need everyone's permission if you only needed a majority, or if the majority or the people collectively decided that delegation of objects to autonomous groups was preferred to the logistical nightmare they faced. There are so many possibilities simply ignored (and the few he explicitly reject are those which nobody believes in). It might puzzle you why Hoppe, a libertarian, is so concerned about getting permission from others to do things involving their property until you hear him talk about private cities and the need to physically remove undesirables, where he definitely wants those "high time preference" blacks, hippies, gays, and communists to need his permission to exist in his community. Other examples are apparent in how he fails to address other values besides argumentation that might take precedence over argumentation, the earlier mentioned way he analyzes only a narrow handful of other possible forms property could take (nonphysical and collective), or other possibilities like that property, even if we grant it to be necessary in some form, is still a negotiated social construct and could only be the product of argument rather than a premise of it (although he probably convinces more people by not bringing up obvious truth that contradicts his narrative).
You could probably consider all of these problems to be symptoms of the larger systemic problem that Hoppe's argument is basically to just presuppose that anarcho-capitalism is the only option, then show how every alternative besides anarcho-capitalism contradicts anarcho-capitalism (how easy!) and thus is incorrect.
To conclude with his own logic, his sequence of "a priori true is-statements", his second statement...
argumentation presupposes property in one’s body and the homesteading principle
...is not only untrue, it used value-laden premises and is not even properly an is-statement in the way he supports it (only in its grammar, but grammatically, "gay is evil" is an is-statement).
So my conclusion is that AE is a badly-plagiarized argument that fails the is-ought test, appealing only to people who already agree with its conclusions and want to make those conclusions the terms of engagement for argument. This is useless to me. I can't even use this to convince a statist that they are wrong and push them toward something less destructive to my goals. I don't think a single person has been convinced of libertarianism by this argument, only that libertarians have been occasionally convinced that this is a good argument to use. This argument seemingly doesn't even aim at non-libertarians, only at libertarians, just as it has nothing to say to nihilists, only to other moral rationalists.