3. The idea of an international community of law could not have obtained acceptance before a time when there existed a number of completely independent states, internally akin in virtue of a community of intensive civilization and continually brought into contact with one another by a lively intercourse. It was in this way that an international community of law was begotten at the end of the middle ages out of Christian civilization and mutual intercourse. Grotius and his forerunners would not have been able to create international law, had not the conception of a community of law between Christian states enjoyed a general recognition, and had not international intercourse before their day evolved already a large number of rules of intercourse, which were based on custom and in part on very ancient usages. [Sidenote: The law of nature as the basis of the law of nations.] 4. A theoretical basis for the erection of a system of international law was provided by the law of nature. This likewise is duteously traced back by science to Grotius, although in this department also he stands on the shoulders of his predecessors. The riddle, how it was possible to find a foundation for international law (as also for constitutional law and other branches of law) in the law of nature, which itself reposed upon so unstable a basis, is easy of solution for those who contemplate the historical development of all law with minds clear from prejudice. The contention of the historical school that all law springs up 'naturally', like language, is chimerical. Wherever a demand for law and order imperiously asserts itself, rules of law arise there. Every epoch of history produces alike that mode of legal development which it needs and that theoretical basis therefor which corresponds to its own interpretation of the nature of things. Accordingly the growth of law is everywhere dependent on, or at least influenced by, a conscious or unconscious creation of law. Custom, usage, habit, religion, morality, the nature of the thing, tradition, reason, the examples of single individuals, and many other factors, contribute the material out of which the requisite rules of law are built up. Where a strong central authority busies itself, year in year out, with legislation, expressly enacted law naturally takes the foremost place, and customary law makes itself felt to a less and less degree. But where such a strong central authority does not exist or does not busy itself with continuous legislation, then the above-named factors exercise a more direct influence upon the development of law, should there arise in actual life an imperious demand for definite rules of law. The theory of natural law was only the mirror held up by legal philosophy, in which the rays emitted by these factors were focused into a homogeneous image. [Sidenote: Positive international law.]