Community is threefold: household or family, city, and realm. A household is a community coming together from those things that make for common actions; thus it consists of three conjunctions, from father and son, from husband and wife, from lord and servant. The community of the city contains all things that are necessary for human life: thus it is a complete community with respect to merely necessary things. The third community is the realm, which is the community of consummation. For where there is fear of foes, a city cannot subsist through itself; thus from fear of foes a community of many cities, which make up one realm, is necessary. Thus as life is in a man, so peace is in a realm; and just as health is nothing but moderating of humors, so peace is when each keeps to his own order. And as when, losing health, the man tends to destruction, so it is with peace: as it dissipates from the realm, the realm tends to destruction. Thus the ultimate thing to which one tends is peace. Thus the Philosopher: 'As the doctor to health, so the defender of the republic to peace.'
Thomas Aquinas, Commentary on the Gospel of Matthew, (C. 12, L. 2, n. 1011), my translation.
