To decide that the son of a slave is born a slave is to decide that he is not a man.
Gist of Idea
We can never assume that the son of a slave is a slave
Jean-Jacques Rousseau (The Social Contract (tr Cress) , IV.2)
Obviously this is because men are 'born free', though I am not clear how that maxim can be reached. I take it for granted that African slaves in the Americas found themselves born into slavery. No justification was required.
Rousseau,Jean-Jacques: 'The Basic Political Writings', ed/tr. Cress,Donald A. [Hackett 1987], p.205