"Contradictory convention that sets up, on the one side, absolute authority, and, on the other, unlimited obedience." What does this mean?

It means that slavery is a tyrannical system.

Explanation:

This sentence was written by Jean-Jacques Rousseau and was aimed at explaining slavery as something tyrannical and suffocating, which does not allow the responsibility and the rights of each man over himself, because it makes some men have too much power over others, while other men have no alternative but to completely obey the powerful, losing their freedom, autonomy and responsibility over themselves and their own actions.

council members were chosen randomly, and men could present laws for debate in assembly.