Keeping Kings in Check 79wrote a major work of po liti cal theory, Defensor pacis (Th e Defender of
Peace), in 1324. Th e discovery that he was the author of this treatise (which
strongly contested the right of the clergy to exercise po liti cal power) led to
his exile at Louis’s court in Bavaria. For present purposes, Marsilius is no-
table for the pervasive infl uence of Aristotelian po liti cal ideas in his work.
Echoing his ancient pre de ces sor, Marsilius regarded the state as a natural
phenomenon, rather than as an evil made necessary by human depravity.
Refl ecting his medical background, he likened the state to a living organism
with a lifestyle consisting of successive stages.
More than any other medieval writer, Marsilius dispensed with the idea
of natural law. He did not reject the concept in principle, but he recognized
it only as “the science or doctrine of right” and not as actual law. Law in the
proper sense, he insisted, is “a command coercive through punishment or
reward... in the present world.” Th e fundamental source of law, accord-
ingly, is not reason but the will of the party promulgating it. Moreover, an
indispensable sign of law is the omnipresent exposure to a sanction or pun-
ishment for its violation— just what natural law lacked. Th is is the clearest
expression in the medieval era of what would later be termed the positivist
conception of law.
Th ese Aristotelian ideas were unsettling for various reasons. For one thing,
they made it relatively diffi cult to see how there could be law between states,
if states were regarded as, in principle, entirely in de pen dent of one another.
If there is no ruler of the body of states, how can there be any law between
them? Th e answer, of course, was that even states that are in de pen dent of
one another do not live in a normless world. Th ey are still subject to the law
of nature and to the ius gentium. But the Aristotelian outlook was neverthe-
less worrying in that, if these are regarded as the only constraints operating
on states, then, in the absence of any enforcement or sanctioning mecha-
nism, it is hard to see the international rule of law as being anything more
than a scholarly reverie.
Th at the international rule of law was a tender plant could hardly be denied
(then or now). But it had some strengths in places that were not so obvious.
Apart from the scholars, practical men were at work, too— not in monastic or
university libraries, but out on the highways and sea- lanes of medieval Eu rope
(and beyond). Th ese adventurers faced many hazards— but they also showed
an impressive degree of legal creativity in devising ways to reduce them.