States' rights
The Constitution did not even mention the word "slave". The framers of the Constitution, many of them slave owners, wanted to avoid the problem at the federal level. The only reference was to the three-fifths rule, which counted three-fifths of a state's slave population which gave a state extra representation, extra votes in the Electoral College, and an increase in its direct taxes. There, the text said not "slaves" but used the expression "all other persons" to mean the same thing.