A law, properly speaking, regards first and foremost the order to the common good. Now to order anything to the common good, belongs either to the whole people, or to someone who is the viceregent of the whole people. And therefore the making of a law belongs either to the whole people or to a public personage who has care of the whole people: since in all other matters the directing of anything to the end concerns him to whom the end belongs.
Thomas Aquinas, ST 1-2.90.3
Accordingly, the best form of government is in a state or kingdom, where one is given the power to preside over all; while under him are others having governing powers: and yet a government of this kind is shared by all [ad omnes pertinet], both because all are eligible to govern [ex omnibus eligi possunt], and because the rulers are chosen by all [ab omnibus eliguntur]. For this is the best form of polity, being partly kingdom, since there is one at the head of all; partly aristocracy, in so far as a number of persons are set in authority; partly democracy, i.e. government by the people [potestate populi], in so far as the rulers can be chosen from the people [ex popularibus possunt eligi principes], and the people have the right to choose their rulers [ad populum pertinet electio principum].