The Unconstitutionality of Slavery (1845) was a book by American abolitionist Lysander Spooner arguing that the United States Constitution prohibited slavery.^[1][2] This view was contrary to that of William Lloyd Garrison who opposed the Constitution on the grounds that it was pro-slavery. In the book, Spooner shows that none of the state governments of the slave states specifically authorized slavery, that the U.S. Constitution contains several clauses that are inconsistent with slavery, that slavery was a violation of natural law, and that the intentions of the Constitutional Convention have no legal bearing on the document they created. Thus, Spooner's position is one that employs original meaning-styled textualism and rejects original intent-styled originalism.

On May 23, 1851, in Change of Opinion Announced, Frederick Douglass attributed his shift in opinion away from Garrison's view that the Constitution was pro-slavery "to Lysander Spooner, Gerrit Smith, and William Goodell. Of all these sources, Spooner likely had the strongest influence on Douglass's method...."^[3]