In the 20th century, the American philosopher Robert Nozick defended the notion of voluntary slavery, whereby persons voluntarily sell themselves into slavery. In "Anarchy, State and Utopia", Nozick writes that "The comparable question about an individual is whether a free system will allow him to sell himself into slavery. I believe that it would." Commenting on Nozick's views, David Ellerman (writing under the pseudonym "J. Philmore") notes their parallels with provisions in the Institutes of Justinian which permit individuals to sell themselves into slavery.