Here was my short critique of the estoppel argument:
One of the more recent attempts to provide a foundation for punishment and proportionality has been what Stephen Kinsella calls ―the estoppel argument. Kinsella has summarized the estoppel argument as follows:
“I will thus seek to justify punishment exactly where it needs to be justified: at the point at which we attempt to inflict punishment upon a person who opposes the punishment. In short, we may punish one who has initiated force, in a manner proportionate to his initiation of force and to the consequences thereof, exactly because he cannot coherently object to such punishment. It makes no sense for him to object to punishment, because this requires that he maintain that the infliction of force is wrong, which is contradictory because he intentionally initiated force himself. Thus, he is estopped, to use related legal terminology, or precluded, from denying the legitimacy of his being punished, from withholding his consent.”
The first thing to point out about this line of argument is that it seems to presume precisely what it must prove, namely, that the individual in question cannot or does not have a rational argument against punishment. It does not make sense to proclaim that the very act of the individual arguing implicitly justifies Kinsella‘s argument; that would be little more than a rhetorical trick. We must note that Kinsella‘s statement here simplifies the non-aggression principle and assumes it as a common starting point with the same meaning or implication to all parties. At best, his argument may intuitively make sense in that it would be hypocritical for someone to murder someone and then demand not to be murdered.
Yet the problem with this argument is that it falls prey to itself, via something we have pointed out already in our exposition of the issue of punishment and proportionality: veiled behind the rhetoric of the argument, the proponent of the estoppel argument is trying to claim that the initiation of force is wrong and that it is justified for them (or some other individual or group) to initiate force at the same time, because someone else has initiated force. In other words, it could equally be argued that the proponent of punishment is being a hypocrite because they are claiming the right (or the right of some 3rd party of punishment enforcers) to aggress (not in self-defense) while simultaneously trying to make a non-aggression argument. Hence, the proponent of the estoppel argument could likewise be ―estopped‖. While the estoppel argument is an attempt at justifying a particular view of justice by making use of consistency, it is not consistent itself.