Chapter IX of Natural Law and Natural Rights begins with a reflection on the need and justification for authority. Finnis writes:
[T]he greater the intelligence and skill of a group’s members, and the greater their commitment and dedication to common purposes and common good, the more authority and regulation may be required, to enable that group to achieve its common purpose, common good.
For . . . the dedicated member of the group will always be looking out for new and better ways of attaining the common good, of co-ordinating the action of members, of playing his own role. And the intelligent member will find such new and better ways, and perhaps not just one but many possible and reasonable ways. Intelligence and dedication, skill and commitment thus multiply the problems of co-ordination, by giving the group more possible orientations, commitments, projects, ‘priorities’, and procedures to choose from. And until a particular choice is made, nothing will in fact be done. . . .
There are, in the final analysis, only two ways of making a choice between alternative ways of co-ordinating action to the common purpose or common good of any group. There must be either unanimity, or authority. There are no other possibilities.
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Now there is no need to labour the point that unanimity about the desirable solution to a specific coordination problem cannot in practice be achieved in any community with a complex common good and an intelligent and interested membership. . . .
John Finnis, Natural Law and Natural Rights, IX.1.