Goodell agreed with Lysander Spooner that the Constitution, properly understood, is antislavery. In 1845—shortly before Spooner published the first part of The Unconstitutionality of Slavery—Goodell defended this position in American Constitutional Law in its Bearing upon American Slavery. Despite reaching the same conclusion, Goodell later claimed that he and Spooner differed significantly in how they reached this conclusion: Spooner relied on legal technicalities, whereas Goodell grounded his arguments in first principles. This was scarcely a fair representation of Spooner’s arguments, which were based on natural law and natural rights; these played essential roles in Spooner’s theory of constitutional interpretation. But there was a significant difference nonetheless. The Christian Goodell located the ultimate foundation of natural law and natural rights in the will of God. As he wrote in American Constitutional Law, “There is neither legitimate authority, nor binding precedent, nor valid law, except in harmony with His will.”

Spooner, in contrast, was a deist who rejected all forms of revealed religion. He based his theory of natural law and natural rights on the nature of man and on the necessary conditions of peaceful social interaction. Of course, since deists believed in a creator God, they agreed with Christians that God created human nature as it presently exists, but they did not believe that our obligation to obey the precepts of natural law flows from the will of God. Something is not good because a god wills it to be so. A natural‐​law moral principle is good because, based on the nature of man, it is good for human beings. Moreover, as the qualifier “natural” also suggests, natural‐​law principles are knowable to man’s natural faculties. We may know the principles of justice through reason alone, without appealing to God’s will through the Bible, divine inspiration, or some similar means. This deistic reliance on reason alone meant that Spooner could not weaken the logical unfolding of his theory of justice by citing biblical injunctions to obey governments, nor could he possibly view government as a divinely mandated institution. Free of the potential obstacles posed by revealed religion (especially the Bible), Spooner pushed his theory of natural law and natural rights as far as reason would take him; and this imparted a radical, anarchistic edge to his political theory that most of his classical liberal colleagues (including many abolitionists) and predecessors were unwilling to accept.

Spooner opened The Unconstitutionality of Slavery with a chapter on natural law. A more complete discussion appears in Natural Law, which was originally printed in Benjamin Tucker’s anarchist periodical Liberty and later reprinted as a separate pamphlet of 20 pages. In this and in other later treatments we find a somewhat more radical treatment of natural law that Spooner presented in The Unconstitutionality of Slavery, but these exhibit a different emphasis rather than a shift in doctrine. Throughout his various discussions, Spooner argued that natural law and its concrete expression in terms of natural rights (which receive an important treatment in A Letter to Thomas F. Bayard, 1882), are the only legitimate basis for political obligation. Human legislation has value only insofar as it specifies objective procedures for the enforcement of natural rights. Legislators are incapable of changing the natural law in any fashion, so to the extent that they pass laws that violate natural rights, those laws have no moral authority whatsoever. As Spooner put it in Natural Law (p. 20):