An Extract from Book V of Aristotle's Politics

"Democracy, for example, arises out of the notion that those who are equal in any respect are equal in all respects; because men are equally free, they claim to be absolutely equal. Oligarchy is based on the notion that those who are unequal in one respect are in all respects unequal; being unequal, that is, in property, they suppose themselves to be unequal absolutely. The democrats think that as they are equal they ought to be equal in all things; while the oligarchs, under the idea that they are unequal, claim too much, which is one form of inequality. All these forms of government have a kind of justice, but, tried by an absolute standard, they are faulty; and, therefore, both parties, whenever their share in the government does not accord with their preconceived ideas, stir up revolution."

"That a state should be ordered, simply and wholly, according to either kind of equality, is not a good thing; the proof is the fact that such forms of government never last. They are originally based on a mistake, and, as they begin badly, cannot fail to end badly. The inference is that both kinds of equality should be employed; numerical in some cases, and proportionate in others."

"Still democracy appears to be safer and less liable to revolution than oligarchy...And we may further remark that a government which is composed of the middle class more nearly approximates to a democracy than to oligarchy, and is the safest of the imperfect forms of government."

"Inferiors revolt in order that they may be equal, and equals that they may be superior. Such is the state of mind which creates revolutions."

for more see Aristotle, Politics, Book V