political writer of the Middle Ages should emphasize the necessity of
unity in the political field, the harm of plurality of rulers, the
dangers and damaging effects of demagogy. The good of the state, says
St. Thomas Aquinas, is unity. And who can procure unity more fittingly
than he who is himself one? Moreover the government must follow, as
far as possible, the course of nature and in nature power is always
one. In the physical body only one organ is dominant--the heart; in
the spirit only one faculty has sway--reason. Bees have one sole
ruler; and the entire universe one sole sovereign--God. Experience
shows that the countries, which are ruled by many, perish because of
discord while those that are ruled over by one enjoy peace, justice,
and plenty. The States which are not ruled by one are troubled by
dissensions, and toil unceasingly. On the contrary the states which
are ruled over by one king enjoy peace, thrive in justice and are
gladdened by affluence.[2] The rule of the multitudes can not be
sanctioned, for where the crowd rules it oppresses the rich as would a
tyrant.[3]

Italy in the Middle Ages presented a curious phenomenon: while in
practice the authority of the state was being dissolved into a
multiplicity of competing sovereignties, the theory of state unity and
authority was kept alive in the minds of thinkers by the memories of
the Roman Imperial tradition. It was this memory that supported for
centuries the fiction of the universal Roman Empire when in reality it
existed no longer. Dante's _De Monarchia_ deduced the theory of this
empire conceived as the unity of a strong state. "Quod potest fieri
per unum melius est per unum fieri quam plura," he says in the XIV
chapter of the first book, and further on, considering the citizen as
an instrument for the attainment of the ends of the state, he
concludes that the individual must sacrifice himself for his country.