The Fugitive Slave clause:
No person held to service or labor in one state, under the laws thereof, escaping into another, shall, in consequence of any law or regulation therein, be discharged from such service or labor, but shall be delivered up on claim of the party to whom such service or labor may be due.
and the famous,
... determined by adding to the whole number of free persons, including those bound to service for a term of years, and excluding Indians not taxed, three fifths of all other Persons.
For the census, they were counted as whole persons. For representation in Congress, three-fifths of their census total was counted.
The peculiar institution recognized a peculiar property right to the labor of an enslaved person, and holding that person in bondage.
1st property, then person.