Smith explains why Spooner believed that defending the unconstitutionality of slavery was essential to abolitionism.

George H. Smith was formerly Senior Research Fellow for the Institute for Humane Studies, a lecturer on American History for Cato Summer Seminars, and Executive Editor of Knowledge Products. Smith’s fourth and most recent book, The System of Liberty, was published by Cambridge University Press in 2013.

As I noted in a previous essay, there are two distinct theoretical strains that run throughout Lysander Spooner’s The Unconstitutionality of Slavery. First and most fundamental is the anarchistic premise that all law, if it is to carry any moral obligation, must be consistent with the natural law of justice. A natural “moral law” “is merely an universal principle of moral obligation, that arises out of the nature of men, and their relations to each other, and to other things—and is consequently as unalterable as the nature of men. And it is solely because it is unalterable in its nature, and universal in its application, that it is denominated law. It is were changeable, partial or arbitrary, it would be no law.”

Spooner’s second perspective was that of a constitutional scholar. Although Spooner could not bring himself to accept the validity of the U.S. Constitution, he nevertheless wrote most of The Unconstitutionality of Slavery from that point of view. According to a strict understanding of Spooner’s position on moral law, the constitutionality of slavery is theoretically irrelevant. As a morally illegitimate document, what the Constitution says about slavery is irrelevant. If it opposes slavery, fine and good, but the injustice of slavery does not depend on its pronouncements. If, on the other hand, the Constitution supports slavery, that also makes no difference from a juridical perspective. Slavery remains a monstrous injustice regardless of what the Constitution may say about it, and no one is morally bound by any constitutional provisions about slavery.

A natural right specifies the moral obligations of natural law as they pertain to the obligations between two or more people. A natural right “necessarily fixes, determines, defines and governs the civil rights of men.” A natural right is “simply the rule, principle, or obligation or requirement of natural justice.”

Regarding this approach to natural law, Spooner wrote:

Such is the true meaning of the term law, as applied to the civil rights of men. And I doubt if any other definition of law can be given, that will prove correct in every, or necessarily in any possible case. The very idea of law originates in men’s natural rights. There is no other standard, than natural rights, by which civil law can be measured. Law has always been the name of that rule or principle of justice, which protects those rights. Thus we speak of natural law.

Spooner pointed out that written (or civil) law cannot cover every detail of every complex case that may arise; and “Wherever the cases have not been thus anticipated, the natural law prevails.” In other words, natural law is the default position to which judges appeal whenever a problem arises that is not covered by a written provision in a legal code.

We thus politically and judicially recognize the principle of law as originating in the nature of rights of men. By recognizing it as originating in the nature of man, we recognize it as a principle, that is necessarily as immutable, and as indestructible as the nature of man. We also, in the same way, recognize the impartiality and universality of its application.

From the foregoing, it follows that natural law is the “paramount law.” It is the universal, fundamental principle to which we appeal when a case is not covered by positive law or when positive laws conflict, so it is the ultimate moral authority in legal matters. No governmental law can possibly overrule the natural law of justice. Whenever a government attempts to do this, it is the legislation of a government, not the natural law of justice, that is rendered void and of no moral effect. No human being, or group of humans calling themselves a “government,” can possibly divest natural rights of their moral authority.

Since natural rights recognize the validity of voluntary contracts, they permit “the formation of a government, founded on contract, as all our governments profess to be.” There are, however, natural moral limits to the provisions of a legitimate social contract.

But in order that the contract of government may be valid and lawful, it must purport to authorize nothing inconsistent with natural justice, and men’s natural rights. It cannot lawfully authorize government to destroy or take from men their natural rights; for natural rights are inalienable, and can no more be surrendered to government—which is but an association of individuals—than to a single individual. They are a necessary attribute of man’s nature; and he can no more part with them –to government or anybody else—than with his nature itself.

I called Spooner’s perspective on natural law and natural rights “anarchistic” because, though he conceded the theoretical possibility of a just government—an institution that would arise if a constitution were completely consistent with the principles of natural law—this hypothetical had never existed in human history and was unlikely ever to be fulfilled. The U.S. Constitution certainly did not meet the necessary requirements, so it possesses no moral authority whatsoever. Nonetheless, Spooner argued as if the Constitution did have the moral authority that its defenders attributed to it. He did this because he believed that his views on the natural law of justice would be ineffective in ending slavery in the United States. To deny the authority of the Constitution in the course of attacking slavery would make the task of abolitionists even more difficult than it already was. Garrison and his followers had already tried this, of course, but by 1845 (when the first part of The Unconstitutionality of Slavery was published) the influence of the Garrisonians was waning, and members of the Liberty Party seemed to have the advantage. In Appendix B of his tract (“Suggestions to Abolitionists”), Spooner explained the tactical advantage of maintaining that slavery is unconstitutional.

Those who believe that slavery is unconstitutional, are the only persons who propose to abolish it. They are the only ones who claim to have the power to abolish it. Were the entire North to become abolitionists, they would still be unable to touch the chain of a single slave, so long as they should concede that slavery was constitutional. To say, as many abolitionists do, that they will do all they constitutionally can towards abolishing slavery, is virtually saying that they will do nothing, if they grant, at the same time, that the constitution supports slavery. To suppress the slave trade between the States, as some propose, is certainly violating the spirit, and probably the law, of the constitution, if slavery be constitutional. To talk of amending the constitution, by the action of three fourths of the States, so as to abolish slavery, is to put off the matter to some remote and unknown period. While abolitionists are amusing themselves with these idle schemes for abolishing slavery without the agency of any adequate means, slaves are doubling in numbers every twenty‐​five years, and the slave power is rapidly increasing in numbers, wealth, and territory. To concede that this power is entrenched behind the constitution, is, in the minds of practical men, to concede the futility of all efforts to destroy it. And its effect is to dissuade the great body of the North from joining in any efforts to that end. The mass of men will insist upon seeing that a thing can be done, before they will leave the care of their other interests to assist in doing it. Hence the slow progress of all political movements based on the admission that slavery is constitutional.

Spooner further maintained that southern lawyers, who typically upheld a strict constructionist view of constitutional interpretation, would never change their view of abolition so long as they believed that the Constitution is proslavery. Spooner therefore provided them with a text that argues that the Constitution, strictly interpreted, is antislavery.

Spooner’s arguments for the unconstitutionality of slavery are varied and complex, but they all depend on a common core, viz., his “rules of interpretation.” Spooner discusses 14 of these rules in the second part of The Unconstitutionality of Slavery, but in the first part he discusses only one.

I shall only claim that in the interpretation of all statutes and constitutions, the ordinary legal rules of interpretation be observed. The most important of these rules, and the one to which it will be necessary constantly to refer, is the one that all language must be construed “strictly” in favor of natural right.

This rule treats natural right as a presumption, a beacon to guide legal interpretations. We must treat all language in the Constitution as if it were consistent with natural rights, unless there is clear and convincing evidence to the contrary. Before we may interpret a constitutional provision as contrary to natural right (i.e., as upholding slavery), the terms of that provision must be “express, explicit, distinct, unequivocal, and one to which no other meaning can be given.

Spooner falls back on this basic rule of interpretation while examining the supposed slavery clauses in the Constitution. Any apparent call to violate natural rights must be stated explicitly and not permit another, more libertarian interpretation. The slavery clauses in the Constitution do not pass this crucial test, according to Spooner.