We need not, it is true, believe that it was logically formulated in primitive times and ruthlessly applied.  Some of its applications were the result of positive legislation due to a growing consciousness of the self-sufficiency of the city state and of the privileges of citizenship, as when Athens passed a law excluding from citizenship the offspring of citizens who had married foreign wives.  But in its broad outlines the principle is sufficiently borne out by the exceptions which were necessary to make human intercourse possible.  The stranger within your gates is protected just because he is within your gates, and you throw your protection about him, as is indeed your duty, for suppliants and strangers come from Zeus.  The foreigner, even at a distance, may have a citizen as representative who can and will defend his rights.  A stranger may be allowed to take up a permanent residence in the city, and by the mediation of a patron or guardian enjoy private rights not much inferior to those of a citizen.  His legal position will not be very different from that of a woman citizen, who needs the like mediation.  Cities may, again, by treaty confer on each other’s citizens reciprocal rights of legal protection.

In the middle of the third century B.C., Rome, after its first successful war against Carthage, took special measures to deal with the problem of the alien litigant.  The great and growing commerce which came from all parts of the Mediterranean called for something more than a mere admission to treaty privileges.  A special officer was from henceforth appointed to deal with the law-suits to which foreigners were parties, and the judgement was given by a body (which we may compare with our jury) which might include fellow-citizens of the foreign suitor.

But here a difficulty arose:  what law was to be applied to a transaction between a Roman and a foreigner, or between two foreigners?  The Roman law, the law of citizens, had been codified two centuries earlier, and its outline had been hardened by the practice of two centuries.  The forms for a transfer of property, for instance, were rigid and solemn; the foreigner would hardly know them, and if he did, his alien hand could not effectively do the prescribed acts nor his alien mouth speak the almost sacred words.  The answer was that behind the forms of the law of this city or that, there was ‘a law of the men of all nations’.  The common elements in the ordinary transactions of life, in whatever form they were clothed, could be taken into account and given effect to.  Thus, side by side with the ownership according to the law of Roman citizens, the solemn words of promise which only a Roman citizen could utter, the marriage which only a Roman citizen could enter into, there might be property, contract, marriage to which any one, citizen or alien, might be a party.