Smith provides some background on Spooner’s influential book The Unconstitutionality of Slavery.

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.

Lysander Spooner’s The Unconstitutionality of Slavery was one of the most widely circulated and read books written by an abolitionist. It was published in two parts—the first in 1845 and the second in 1847. Part II followed on the heels of a critique of Part 1 by Wendell Phillips, Review of Lysander’s Spooner’s Essay on the Unconstitutionality of Slavery (1847), and much of Part II contains Spooner’s replies to Phillips. Both parts were frequently reprinted, and they were published as a single book in 1847, 1849, 1853, 1856, and 1860. The 1860 printing contains two additional essays, including A Defence for Fugitive Slaves (1850)—Spooner’s devastating critique of the notorious Fugitive Slave Act of 1850, which prompted many antislavery activists to use violence to rescue runaway slaves from their captors.

Before the publication of The Unconstitutionality of Slavery, the major work on this topic was William Goodell’s Views of American Constitutional Law and Its Bearing Upon American Slavery­, published in 1844. (Another one of Goddell’s books, Slavery and Anti‐​Slavery—a history of the antislavery movement published in 1852—is still highly regarded by historians.) But Goodell’s approach differed from that of Spooner. Goodell emphasized the sinfulness of slavery, whereas Spooner focused on legal and constitutional issues. In a letter to Spooner, Goodell wrote that Spooner used the language of the courts, whereas he used the language of the pulpit: “you speaking of ‘natural law’ and I of the ‘will of the Creator.’—You quoting legal authorities and I the Hebrew Prophets.” Most abolitionists regarded The Unconstitutionality of Slavery as the superior book by far. William Lloyd Garrison claimed that Spooner had replaced Goodell as the “oracle” of those abolitionists who believed slavery to be unconstitutional.

Phillips, Garrison, and some other anti‐​political critics of Spooner praised Spooner’s efforts in some respects. Phillips said that Spooner had defended his position with “unrivalled ingenuity, laborious research, and close logic.” Similarly, Garrison wrote this about one of Spooner’s arguments:

His logic may be faultless, as a mere legal effort. We admit Mr. Spooner’s reasoning to be ingenious; perhaps, as an effort of logic, unanswerable. It impresses us as the production of a mind equally honest and acute. Its ability, and the importance of the subject on which it treats, will doubtless secure for it a wide circulation and a careful perusal.

Spooner’s book came as a godsend to Gerrit Smith and other members of the abolitionist Liberty Party, who were badly in need of competent legal arguments to defend their position that the U.S. Constitution, properly understood, was antislavery. Smith, a wealthy New York landowner and philanthropist, financed Spooner so he could write the book. And Smith was overjoyed when he read it.

The more I read that admirable, invincible, and matchless argument, which Lysander Spooner has made to show the unconstitutionality of slavery, the more I am pleased with it. He yields nothing but what the legal rules of interpretation compel him to yield. And why should he make unnecessary concessions in an argument undertaken in behalf of all that is sacred and vital in the rights of man? Were I studious of fame or usefulness, I had rather be the author of this manly, brave, and independent argument against the constitutionality of slavery, than of any other law argument ever written, either in this age, or in any former age—either on this side, or on the other side, of the Atlantic. Why will not all lawyers read it? Who of them could read it without being convinced that slavery is unconstitutional?

In 1849 the Liberty Party expressed its gratitude to Spooner and its desire to distribute his book to every attorney in the United States.

Whereas, Lysander Spooner, of Massachusetts, that man of honest heart and acute and profound intellect, has published a perfectly conclusive legal argument against the constitutionality of slavery;

Resolved, therefore, that we warmly recommend to the friends of freedom, in this and other States, to supply, within the coming six months, each lawyer in their respective counties with a copy of said argument.

In subsequent essays I shall explore Spooner’s specific arguments; for the remainder of this essay I shall confine myself to some general remarks.

Spooner’s entire case for the unconstitutionality of slavery was predicated on the idea of natural law and natural rights. He argued that something is not truly a “law” unless it is consistent with these notions. There are two potential problems here that I should attempt to clear up.

For centuries philosophers have distinguished natural law from positive law. The latter signifies an enactment by a sovereign government, whether that government be limited or absolute, and whether the enactment be just or unjust. Thus we say that Hitler’s Germany passed and enforced many laws, however evil and unjust those laws may have been. But Spooner criticized this conventional meaning of “law.” He maintained that “law,” properly considered, refers to a universal, invariable rule, as we see when we speak of the “laws” of physics, biology, and so forth. Thus the “laws” that govern human beings and their social interactions must also be universal and invariable. But the “laws” decreed by governments are nothing of the sort. They vary dramatically from one nation to the next, and current laws are frequently repealed and new laws enacted. According to Spooner, only natural laws—fundamental rules of conduct, expressed in terms of natural rights, that flow from the nature of human beings—should properly be called “laws.” Natural rights are the standard by which we assess the justice or injustice of governmental enactments. Here is how Spooner summarized his perspective:

I shall define [law] to be simply the rule, principle, obligation or requirement of natural justice.

This rule, principle, obligation or requirement of natural justice, has its origin in the natural rights of individuals, results necessarily from them, keeps them ever in view as its end and purpose, secures their enjoyment, and forbids their violation. It also secures all those acquisitions of property, privilege and claim, which men have a natural right to make by labor and contract.

I raise this issue here, before explaining Spooner’s position in more detail, because it is necessary to understand that Spooner was well aware that many people do not restrict the meaning of “law” solely to natural laws. Spooner’s basic point was that only natural laws carry moral authority. The fact that a government passes a “law” and demands that people obey it lacks all such moral authority in its own right; any authority is may have derives entirely from its consistency with natural rights. Human laws that violate rights are based on nothing but brute force, and we have no moral obligation to obey them. Of course, we might obey the unjust decrees of a government from fear of being punished by force, but this is a matter of prudence, not morality. As John Locke had argued centuries earlier, force alone cannot create right. The fact that I am pointing a gun at you may compel you to obey me, but my threat does not morally obligate you to obey me. And, Spooner argued, as it goes with individuals, so it goes with governments, which are nothing but collections of individuals who presume to tell others how to live.

A second source of possible confusion is Spooner’s equating of moral laws with physical laws. In the first chapter of The Unconstitutionality of Slavery, he wrote:

The true and general meaning of [law], is that natural, permanent, unalterable principle, which governs any particular thing or class of things. The principle is strictly a natural one; and the term applies to every natural principle, whether mental, moral or physical. Thus we speak of the laws of mind; meaning thereby those natural, universal and necessary principles, according to which mind acts, or by which it is governed. We speak too of the moral law; which is merely an universal principle of moral obligation, that arises out of the nature of men, and their relations to each Edition: current; Page: 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. If it were changeable, partial or arbitrary, it would be no law. Thus we speak of physical laws; of the laws, for instance, that govern the solar system; of the laws of motion, the laws of gravitation, the laws of light, &c., &c.—Also the laws that govern the vegetable and animal kingdoms, in all their various departments: among which laws may be named, for example, the one that like produces like. Unless the operation of this principle were uniform, universal and necessary, it would be no law.

Every undergraduate philosophy student will have a ready response to this passage: A physical law expresses what is, whereas a natural moral law expresses what ought to be. A volitional human being does not necessarily follow moral law, as a brick follows the physical law of gravitation when we drop it. We may say that a person should respect the rights of others, but we cannot say that all people do in fact respect the rights of others, since many people violate those selfsame rights. People are not bricks; people have the power to make choices.

This of course relates to the classic Is‐​Ought Problem so beloved by modern moral philosophers. I cannot possibly discuss the problem here, except to note that I do not regard it as an insurmountable obstacle to a natural‐​law ethics of the sort that Spooner defended. (I may have more to say about this in subsequent essays.) Was Spooner aware of this problem? I think he almost certainly was, as any legal philosopher would have been after Jeremy Bentham wrote his celebrated attacks on a natural‐​law ethics, including the doctrine of natural rights, decades earlier. (Even many philosophers seem not to understand that, according to Bentham, we should not apply the term “law” even to physical nature. It should be confined solely to the enactments of a sovereign government.)

If Spooner was aware of this problem, then why didn’t he discuss it? The reason, I suspect, is because Spooner was writing a legal treatise, not a tract on moral theory. Spooner was writing primarily for lawyers and judges for whom the natural‐​law basis for a just legal system was taken for granted. Few if any American legal philosophers in Spooner’s day would have disagreed with Spooner on this matter. It was therefore unnecessary for Spooner to defend a doctrine with which the vast majority of his readers already agreed.