The chief of these is the Greek idea of the city-state, which reached its highest development in Athens. Citizenship was, as a rule, hereditary, and every possible legal measure was taken to preserve its purity. The main principle of this hereditary citizenship was that the union from which the child was sprung must be one recognized by the State. This was accomplished by requiring a legitimate marriage, either through betrothal by a parent or guardian, or through assignment by a magistrate. Pericles revised the old conditions, which had become lax during the tyranny, by passing a measure limiting citizenship to those who were born of two Athenian parents. Greater stress was laid on the citizenship of the mother than on that of the father, as the child was regarded as belonging naturally to the mother. It was possible to increase the citizen body by a vote of the people; but in the best days of Athens her citizenship was regarded as so high a privilege that the franchise was most jealously guarded. Consequently, in the fifth century we see in Athens and Attica a population of about four hundred thousand, of which not more than fifty thousand were citizens; the rest consisted of minors, of resident aliens numbering some fifteen thousand, and of slaves, of whom there were about two hundred thousand in the Periclean Age. To preserve the purity of the citizenship in so large a population of residents, increased by thousands of visitors and strangers who frequented the metropolis, every precaution was taken that the daughters of Athens should not be wedded to foreigners, and that no spurious offspring should be palmed off on the State. Hence marriage by a citizen was restricted to a union with a legitimate Athenian maiden with full birthright. The marriage of an Athenian maiden with a stranger, or of a citizen with a foreigner, was strictly forbidden, and the offspring of such a union was illegitimate. Under such a conception of polity, marriage lay at the very basis of the State; and respect for the local deities, obligations of citizenship, and regard for one's race and lineage, demanded that every safeguard should be thrown about it, and that the women of Athens should conform to those enactments and customs which would fit them to be the mothers of citizens and would keep from them every entangling intrigue with strangers.