Slavery, if it can be legalized at all, can be legalized only by positive legislation. Natural law gives it no aid. Custom imparts to it no legal sanction.

In this quote, Lysander Spooner, a 19th-century political theorist and abolitionist, argues that slavery cannot be justified by natural law or by any longstanding custom or social practice. Instead, he claims that if slavery were to be legalized, it would require positive legislation, meaning it would need to be explicitly written into law by government authorities. Spooner is emphasizing that, in a moral and legal sense, natural law—the idea that certain rights are inherent by virtue of human nature—does not support the institution of slavery, nor does custom (the acceptance of slavery over time) give it any legitimate legal sanction.

Spooner’s statement challenges the prevailing notion that slavery could be justified by tradition or the customs of a society. He contends that despite its long history, slavery does not have any inherent legal validity. By arguing that only positive legislation—laws specifically enacted by a legislative body—could make slavery legal, Spooner rejects the idea that customs or moral frameworks can provide a legitimate basis for such a system. This reflects his deep belief in individual rights and his view that government-imposed systems of injustice can only be justified through clear, deliberate laws.

The quote is deeply connected to Spooner’s abolitionist views and his commitment to natural rights. As a proponent of freedom and individual liberty, Spooner saw slavery as an affront to these principles, which he argued were self-evident. He believed that no society, under the tenets of natural law, could legitimately enforce slavery or any other form of human oppression. Spooner's work, including his famous essay The Unconstitutionality of Slavery, was pivotal in the intellectual defense of the abolitionist movement.

The origin of this quote is tied to Spooner's broader critique of government and legal systems that uphold injustice. As a lawyer and political theorist, Spooner made a name for himself challenging the legal foundations of slavery, government power, and the Constitution. His work laid the intellectual groundwork for arguments against slavery during the years leading up to the Civil War, and this quote remains a striking expression of his view that laws supporting slavery were not only morally wrong but also lacked natural or customary legitimacy.