Whenever I hear anyone arguing for slavery, I feel a strong impulse to see it tried on him personally. Abraham Lincoln
Fortunately, few people do, these days. Thanks in part to Abe. But although almost no-one argues for slavery, slavery still exists. Outright slavery, but also “grey slavery”, people officially receiving a wage and officially having the freedom to leave, but in practice being in a quasi-slavery position.
Article 4 of the Universal Declaration:
No one shall be held in slavery or servitude; slavery and the slave trade shall be prohibited in all their forms.
The right not to be a slave is the negative version of the right to possess your own body. Those who commit slavery (but also those who steal) act as if the bodies of other people are their property, a property which can be bought and sold. Considering other people as your property diminishes the value and dignity of these other people. Other people should not be considered as a means. As a consequence, we have to give every person the right to be considered as a person before the law (slaves are not considered as persons, they are goods that can be traded).