200 Reason and Its Rivals (ca. 1550– 1815)of Eu ro pe an states and then to induce from them “a theory of the law of na-
tions of Eu rope [which is] general, positive, modern and practical.” He was
careful to explain the utility that such an abstracted general law would have:
that, in the resolution of disputes, it would suffi ce to determine and apply
the contents of this general law, without needing to produce evidence of ac-
tual agreement between states on specifi c rules.
Among the puzzles to which Martens turned his attention was the ques-
tion of how a single state could become legally bound to follow a given course
of conduct in the absence of a treaty obligation. A mere per sis tent practice by
a state could not, on its own, have this eff ect, Martens maintained. But it
could give rise to a presumption that the practice was well founded and rea-
sonable. Even a single act by the state in question could give rise to such a
presumption, which could then be “fortifi ed” over time by further experi-
ence. On the whole, a conjunction of various factors could, in combina-
tion, have the eff ect of promoting a customary practice by a state into a rule
of law. Th ese factors included the natural force of habit, the advantage af-
forded to the state itself by continued adherence to the practice, the desire to
be regarded as a civilized state in the eyes of others, and a fear of countermea-
sures or opposition in case of disavowal.
Martens also gave some attention to the relationship between customary
law and treaties. He contended that an ensemble of specifi c arrangements,
such as a network of bilateral treaties on some subject, could be regarded as
constituting, in the aggregate, “a general convention.” Th is general conven-
tion would be a customary or unwritten rule of law, even though it had its
origin in the treaty practice of states.
If Vattel was more successful than Martens in appealing to a wide audi-
ence, Martens had the higher reputation among lawyers and scholars. In
the nineteenth century, he would be regarded, with at least some reason, as
an important progenitor of the positivist philosophy of international law.
Th at praise is somewhat misplaced (as will be seen), but it provides a telling
indication of the esteem in which he was held. Martens was certainly no
aggressive modernist by temperament. His principal contribution to inter-
national law lay in the useful light that he shed on the pro cess by which in-
ternational law is made. Th is was in contrast to Vattel, who remained much
more in thrall to the idea of natural law as an eternal and unchangeable
corpus of rules. Martens was more sensitive the idea that international law