Aristotle embarks on his discussion of the preservation of oligarchies by making about this form of constitution essentially the same point as he made about democracy in the preceding chapters, namely that an extreme oligarchy is less stable than a moderate one. Hence oligarchs have an interest in being to some extent non-oligarchical: they should allow fairly large numbers of the population to ‘share in the constitution’, in order to ensure strong enough support for the perpetuation of the regime.
1320b18 How oligarchy should be handled is fairly clear from the principles already stated. Working from opposites, we must draw up a list of oligarchies, working out each in relation to the democracy opposite to it. We begin with the first, which is also the best mixed and very near to what we call polity. In it division of property-qualifications is necessary, so that some are low and some high, and so that the qualification for holding essential offices is low, but higher for the more sovereign ones. Anyone who reaches the required assessment of property is entitled to share in the constitution; by this means enough of the people are brought in by the assessment to ensure that they are stronger than the non-sharers. But these new sharers must always be drawn from among the better sort among the people. Similarly with the organization of the next type of oligarchy, except that the reins are drawn a little tighter.
1320b30 Finally, there is the oligarchy which corresponds to the extreme democracy, and is most like a power-group and most like a tyranny; it is also the worst, and there is proportionally greater need to watch it. For just as our bodies, if they are in a healthy condition, or boats if they are in proper trim for their crews to sail them, can tolerate errors without being destroyed by them (whereas bodies in a sickly condition and boats with loose timbers and incompetent crews are seriously affected by even minor mistakes), so it is with constitutions: the worst of them need most watching.
1321a1 Generally speaking, then, in democracies a large population1 is a safeguard, just because weight in numbers is the counterpart of the principle of justice which allows weight to merit. But an oligarchy can on the contrary expect to secure its preservation only by enforcing good order.