Slavery was an essential element of the society of Ancient Greece. Social life, in numerous ways -- family, commerce, politics, was heavily dependent on a class of people who fulfilled tasks their masters saw as degrading. Although, the concept of slavery represented a fundamental aspect of the practical construction of democracy, it is not the only factor that has lead to the development of this complex political institution which is currently the basis of modern constitutional systems. Other issues that need to be considered are the developments in philosophy, the urbanization of Greece and the extremely large interest that the population manifested towards politics. (After all, the Greeks were as interested in philosophy and politics as much as the Romans were interested in law). Each of this factors was essentially predetermined by slavery, which as a firm establishment, made possible the development of all political systems in ancient Greece, and of democracy in particular.
The Greeks were divided into classes, which formed an elaborate but in the same time clear structure, which included all types of people -- from wealthy, influential aristocracy (aristoi = the best) to a lowly, powerless population of slaves. It is ironic that the class of the slaves might be considered as important as the aristocracy, as it was an integral part of Greek society, since the right to own another person was rarely questioned in Greece. After all, slavery in a more primitive form originated in Mesopotamia, reached its classical manifestation in ancient Greece and Rome, continued to exist, under all sorts of forms in the Middle Ages and was extinguished, officially at least, during the 19th and 20th centuries.
In Ancient Greece, a man of distinction should not have performed the tasks that were "fit" for slaves and foreigners (Foreigners constituted the main source of slaves. Wars were often fought to renew the slave population. Warriors who managed to survive and all the civilian population, if any had left, was converted into slaves. Even more, Romans were famous for turning into slaves foreign visitors of Rome, simply because they were not Romans).
Aristotle, who might be considered, if not the greatest, than at least the most balanced of all Greek thinkers, was the perfect exponent of the ideas on which Greek society was founded. The class of artisans was excluded from citizenship because they were "debarred" by their occupation from the characteristic excellence of men. These ideas first appeared in Plato's republic and were also uphold by his disciple Aristotle. In that context, if artisans, who were after all Greeks, were denied of their citizenship right, at least in theory, then the slaves could have expected a much more oppressive treatment.
The concept of virtue (arete) might explain how the Greeks understood the nature of man, citizenship and slavery. It is not that slaves are what they are because they are foreigners, but because they are not worthy of being considered men. The syllogism (at which Aristotle was so good, since he invented the concept) was simple: Foreigners are not worthy. Unworthy human beings are slaves. Therefore, foreigners should be slaves. The same could be applied for some Greeks.
Slavery was still an important contributor to ancient Greek democracy. The use of slaves increased wealth, which allowed individuals to go from subsistence farming to other enterprises, such as commerce or philosophy. The city-state was also a slave-owner (one of the first forms of public ownership). These slaves, referred to as hierodouloi) were responsible for a large part of the bureaucratic activity of the state, as well as for providing a police force. The state applied very sound economical policies: it kept their wages extremely low. ( ... talk about tradition). Therefore, the viability of the state as an entity was greatly increased and the urbanization of Athens and similar states was significantly accelerated.
As far as philosophy is concerned, it was a constant companion of politics in ancient Greece. It was thought that the men who had the capacity to contemplate the meaning of life and the universe were the most adequate to rule and to create the political atmosphere of the city-state. Plato said that 'The state is but man writ large', while Aristotle collected from his journeys and compiled a great collection of constitutions…