Sadly, Jason Kuznicki and Ilya Somin make the critical error of mistaking “secession” for “revolution.” Revolution means to overthrow a political and legal order, while secession is a legal theory—it is the theory that for a state to leave the union is itself within the legal order. It is therefore literally incorrect to say that secession is the “attempt to step outside the legal order.” That’s revolution. The American Revolution was not an attempt at secession—note that the word was virtually never used by the Revolutionary leaders. On the contrary, the term secession came into use in the decades before the Civil War as an attempt to justify a (pseudo-)revolutionary act within the legal order. Lincoln himself commented on this fact in his July 4, 1861 address:
It might seem, at first thought, to be of little difference whether the present movement at the South be called “secession” or “rebellion.” The movers, however, well understand the difference. At the beginning, they knew they could never raise their treason to any respectable magnitude, by any name which implies violation of law. They knew their people possessed as much of moral sense, as much of devotion to law and order, and as much pride in, and reverence for, the history, and government, of their common country, as any other civilized, and patriotic people. They knew they could make no advancement directly in the teeth of these strong and noble sentiments. Accordingly they commenced by an insidious debauching of the public mind. They invented an ingenious sophism, which, if conceded, was followed by perfectly logical steps, through all the incidents, to the complete destruction of the Union. The sophism itself is, that any state of the Union may, consistently with the national Constitution, and therefore lawfully, and peacefully, withdraw from the Union, without the consent of the Union, or of any other state. The little disguise that the supposed right is to be exercised only for just cause, themselves to be the sole judge of its justice, is too thin to merit any notice. With rebellion thus sugar-coated, they have been drugging the public mind of their section for more than thirty years; and, until at length, they have brought many good men to a willingness to take up arms against the government the day after some assemblage of men have enacted the farcical pretence of taking their State out of the Union, who could have been brought to no such thing the day before.
So Prof. Somin is correct that secession is not a category error. But, sadly, he persists in error by saying that the Constitution is “silent” regarding secession. While it is true that the Constitution does not use the word secession (that word having come into common parlance decades after it was written), many of its pre-1865 provisions—from the Guarantee Clause to the Privileges And Immunities Clause—were absolutely incompatible with secession, and the subsequent amendments are even moreso.
Update: A comment I left in response to Kuznicki’s latest post:
I’m sorry, but this just shows how important it is to keep our language precise. Obviously it’s true that the term “secession” is now used to refer to any attempt to separate one corporate entity from another of which it is a discrete part, into two corporate entities. But when we are talking about the American Constitution, then the word “secession” means one particular thing—it means the theory that it is lawful for a state unilaterally leave the union. It is not analogous to foreign constitutions, precisely for the reasons identified in Federalist 39—that the dual-sovereignty federalist system is not analogous to any other nation’s constitutional scheme. Perhaps it would be better to call it “American secession.” But the reason this is important is that secession was invented precisely to avoid questions of whether or not the Confederacy was attempting a revolution. It was invented as a way of rationalizing a state’s attempt to leave the union within the legal order of the U.S. Constitution. It is therefore backwards to say that secession—or “American secession”—is an attempt to step outside the legal order. That’s just what it is not.
This explains why it is also wrong to say that “ultimately, ‘the law says we can’ or ‘the law says we can’t; doesn’t answer the question of whether a given secession is a justified act. To answer it, you may also have to appeal to something outside the law.” Actually, if the law allows secession, then no more justification is required—just as a person wanting to sell his car requires no further justification than that he wants to and it belongs to him. If secession is a legal right, no further rationale is required (at least, vis-a-vis the federal union). But if secession is not legal—that is when further justification is required (and in the case of 1861, is lacking). Only if it is a law-breaking act do we get to the question of whether it is nevertheless justified in some moral sense.
Update 2: Prof. Somin’s response warrants a brief clarification: obviously I do not deny that “[m]any legal activities might also be immoral or unjust,” and that secession for the purpose of perpetuating slavery was both. Of course that is true. (That’s why I added the qualifier “vis-a-vis the federal union.”) My point is that if secession were legally valid, then the state would have the legal authority to secede regardless of its rightness or wrongness, just as one has a legal right to do immoral things. But it is only because secession was and is illegal, that the rightness or wrongness of the act is relevant: because it is then incumbent upon a “seceding” state to justify such an act as an act of revolution (which can justify illegal acts in some cases). That route, however, is not open to the Confederacy, due to the immorality of its acts.
Prof. Somin and I have long differed over what I believe to be his inaccurate use of the term “secession” to refer either to revolution, or to some other forms of legal separation that are permissible under the constitutions of other nations. I won’t reiterate here the reasons why I think why his use of these terms is confusing and leads to perverse conclusions. But I do think it’s important to reiterate that the U.S. Constitution’s system of divided sovereignty is in no sense analogous to Alcoholics Anonymous: the states are not “members” of the federal government in the way that people are members of A.A. Indeed, the states are not, properly speaking, “members” at all, for reasons I’ve explained in this article and this. A closer analogy would be if A.A. purported to absolve its members of membership in the AAA. It would have no standing to do so, and any attempt to do so would violate the contractual rights of the members in question. And, obviously, the guarantee clause would be rendered meaningless surplusage if secession were valid, since a coup by non-republican actors could then escape constitutional enforcement by purporting to secede. Which, of course, is exactly what was attempted in 1861.
Update 3: There is no "contradiction" in saying that, if a state had the legal authority to secede, it would have no need to make a moral case to others for doing so; to have a right to do something means one may do it even if it is immoral. But if secession is illegal, then moral issues become critical, because illegal acts can also sometimes be justified, by moral considerations. For further explanation, see my article. And the states are emphatically not "members" of the Constitution. We, the people ordain the Constitution; not we, the states. This distinction is the single most important concept in all of American constitutional law.