“Precepts for living together are not going to be handed down from on high. Men must use their own intelligence in imposing order on chaos, intelligence not in scientific problem-solving but in the more difficult sense of finding and maintaining agreement among themselves. Anarchy is ideal for ideal men; passionate men must be reasonable. Like so many men have done before me, I examine the bases for a society of men and women who want to be free but who recognize the inherent limits that social interdependence places on them.”
Buchanan’s point is that there is no authority other than us humans and that there’s a tradeoff. Social interdependence cannot be avoided because we necessarily have to cooperate with other humans if we wish to enjoy the gains from trade and the division of labor that it entails. For that we have to arrive at some set of rules that we all agree to abide by through some process of negotiation. These rules will limit our own freedom of action but in exchange for that we will gain greater scope to exercise the freedoms and rights that we do retain.
My favorite example of this is the rule that prohibits driving after drinking some pre-specified amount of alcohol. I would vote for restricting the freedom to drink and drive because I would be able to exercise my freedom to drive secure in the knowledge that drunk drivers are less likely to be on the road.
All these rules that make civilized life possible are not “handed down from on high” but have to be arrived at by us. The first task is to agree on a process that will be used to figure out those rules. That is, we have to come up with a higher-level rule on how rules will be made. That higher-level rule has to be decided on. That’s job #1. Another name for that higher-level is the constitutional level. We have to first figure out a constitution, and the constitution then dictates the process that determine the rules that are to be followed in the post-constitutional periods (which will be several.)
Reasoning from first principles, it becomes clear that the constitution must itself require unanimous consent for guaranteeing non-exploitation of any individual or subset of the population.
 From the preface to The Limits of Liberty: Between Anarchy and Leviathan (1975) by James M. Buchanan.