Even more, citizenship everywhere in the earlier period was a degree to be attained to only after proper education and preliminary military and political training. This not only made some form of education necessary, but confined educational advantages to male youths of proper birth. There was of course no purpose in educating any others. [5] From Figure 4 it will be seen what a small percentage of the total population this included. Education in Greece was essentially the education of the children of the ruling class to perpetuate the rule of that class. Attica almost alone among the Greek States adopted anything approaching a liberal attitude toward the foreign-born; in Sparta, and generally elsewhere in Greece, they were looked upon with deep suspicion. As a result most of the foreign residents of Greece were to be found in Athens, or its neighboring port city (the Piraeus), attracted there by the hospitality of the people and the intellectual or commercial advantages of these cities. After Athens had become the center of world thought, many foreigners took up their residence in the city because of the importance of its intellectual life. Foreigners, though, they remained up to 509 B.C. (See page 40.) Only rarely before this date, and then only for some conspicuous act of patriotism, and by special vote of the citizens, was a foreigner admitted to citizenship. Unlike Rome, which received those of alien birth freely into its citizenship, and opened up to them large opportunities of every kind, the Greeks persistently refused to assimilate the foreign-born. Regarding themselves as a superior people, descended from the gods, they held themselves apart rather exclusively as above other peoples. This kept the blood pure, but, from the standpoint of world usefulness, it was a serious defect in Greek life. [6] Beneath both citizens and foreign residents was a great foundation mass of working slaves, who rendered all types of menial and intellectual services. Sailors, household servants, field workers, clerks in shops and offices, accountants, and pedagogues were among the more common occupations of slaves in Greece. Many of these had been citizens and learned men of other City-States or countries, but had been carried off as captives in some war. This was a common practice in the ancient world, slavery being the lot of alien conquered people almost without exception. The composition of Attica, just before the outbreak of the Peloponnesian War (431 B.C.) is shown in Figure 4. The great number of slaves and foreigners is clearly seen, even though the citizenship had by this time been greatly extended. In Sparta and in other City-States somewhat similar conditions prevailed as to numbers [7] but there the slaves (Helots) occupied a lower status than in Athens, being in reality serfs, tied to and being sold with the land, and having no rights which a citizen was bound to respect.