'Most of those whose object it has been to instruct us methodically concerning such matters, distinguish three kinds of constitutions, which they call kingship, aristocracy, and democracy. Now we should, I think, be quite justified in asking them to enlighten us as to whether they represent these three to be the sole varieties or rather to be the best; for in either case my opinion is that they are wrong. For it is evident that we must regard as the best constitution a combination of all these three varieties, since we have had proof of this not only theoretically but by actual experience, Lycurgus having been the first to draw up a constitution — that of Sparta — on this principle.
Nor on the other hand can we admit that these are the only three varieties; for we have witnessed monarchical and tyrannical governments, which while they differ very widely from kingship, yet bear a certain resemblance to it, this being the reason why monarchs in general falsely assume and use, as far as they can, the regal title. There have also been several oligarchical constitutions which seem to bear some likeness to aristocratic ones, though the divergence is, generally, as wide as possible. The same holds good about democracies. The truth of what I say is evident from the following considerations. It is by no means every monarchy which we can call straight off a kingship, but only that which is voluntarily accepted by the subjects and where they are governed rather by an appeal to their reason than by fear and force. Nor again can we style every oligarchy an aristocracy, but only that where the government is in the hands of a selected body of the justest and wisest men. Similarly that is no true democracy in which the whole crowd of citizens is free to do whatever they wish or purpose, but when, in a community where it is traditional and customary to reverence the gods, to honour our parents, to respect our elders, and to obey the laws, the will of the greater number prevails, this is to be called a democracy.
We should therefore assert that there are six kinds of governments, the three above mentioned which are in everyone's mouth and the three which are naturally allied to them, I mean monarchy, oligarchy, and mob-rule. Now the first of these to come into being is monarchy, its growth being natural and unaided; and next arises kingship derived from monarchy by the aid of art and by the correction of defects. Monarchy first changes into its vicious allied form, tyranny; and next, the abolishment of both gives birth to aristocracy. Aristocracy by its very nature degenerates into oligarchy; and when the commons inflamed by anger take vengeance on this government for its unjust rule, democracy comes into being; and in due course the licence and lawlessness of this form of government produces mob-rule to complete the series. The truth of what I have just said will be quite clear to anyone who pays due attention to such beginnings, origins, and changes as are in each case natural. For he alone who has seen how each form naturally arises and develops, will be able to see when, how, and where the growth, perfection, change, and end of each are likely to occur again. And it is to the Roman constitution above all that this method, I think, may be successfully applied, since from the outset its formation and growth have been due to natural causes.'
Polybius: Histories, Book VI