The Complete Aristotle (eng.)

same class; I mean
to say, from those forms of democracy and oligarchy which are
regulated by law into those which are arbitrary, and
conversely.
VII
    In aristocracies revolutions are stirred up when a few only
share in the honors of the state; a cause which has been already
shown to affect oligarchies; for an aristocracy is a sort of
oligarchy, and, like an oligarchy, is the government of a few,
although few not for the same reason; hence the two are often
confounded. And revolutions will be most likely to happen, and must
happen, when the mass of the people are of the high-spirited kind,
and have a notion that they are as good as their rulers. Thus at
Lacedaemon the so-called Partheniae, who were the [illegitimate]
sons of the Spartan peers, attempted a revolution, and, being
detected, were sent away to colonize Tarentum. Again, revolutions
occur when great men who are at least of equal merit are dishonored
by those higher in office, as Lysander was by the kings of Sparta;
or, when a brave man is excluded from the honors of the state, like
Cinadon, who conspired against the Spartans in the reign of
Agesilaus; or, again, when some are very poor and others very rich,
a state of society which is most often the result of war, as at
Lacedaemon in the days of the Messenian War; this is proved from
the poem of Tyrtaeus, entitled ‘Good Order’; for he speaks of
certain citizens who were ruined by the war and wanted to have a
redistribution of the land. Again, revolutions arise when an
individual who is great, and might be greater, wants to rule alone,
as, at Lacedaemon, Pausanias, who was general in the Persian War,
or like Hanno at Carthage.
    Constitutional governments and aristocracies are commonly
overthrown owing to some deviation from justice in the constitution
itself; the cause of the downfall is, in the former, the
ill-mingling of the two elements, democracy and oligarchy; in the
latter, of the three elements, democracy, oligarchy, and virtue,
but especially democracy and oligarchy. For to combine these is the
endeavor of constitutional governments; and most of the so-called
aristocracies have a like aim, but differ from polities in the mode
of combination; hence some of them are more and some less
permanent. Those which incline more to oligarchy are called
aristocracies, and those which incline to democracy constitutional
governments. And therefore the latter are the safer of the two; for
the greater the number, the greater the strength, and when men are
equal they are contented. But the rich, if the constitution gives
them power, are apt to be insolent and avaricious; and, in general,
whichever way the constitution inclines, in that direction it
changes as either party gains strength, a constitutional government
becoming a democracy, an aristocracy an oligarchy. But the process
may be reversed, and aristocracy may change into democracy. This
happens when the poor, under the idea that they are being wronged,
force the constitution to take an opposite form. In like manner
constitutional governments change into oligarchies. The only stable
principle of government is equality according to proportion, and
for every man to enjoy his own.
    What I have just mentioned actually happened at Thurii, where
the qualification for office, at first high, was therefore reduced,
and the magistrates increased in number. The notables had
previously acquired the whole of the land contrary to law; for the
government tended to oligarchy, and they were able to encroach… .
But the people, who had been trained by war, soon got the better of
the guards kept by the oligarchs, until those who had too much gave
up their land.
    Again, since all aristocratical governments incline to
oligarchy, the notables are apt to be grasping; thus at Lacedaemon,
where property tends to pass into few hands, the notables can do
too much as they like, and are allowed to marry whom they please.
The city of Locri was ruined by a marriage connection with
Dionysius, but such a thing could never have happened in a
democracy, or in a wellbalanced aristocracy.
    I have already remarked that in all states revolutions are
occasioned by trifles. In aristocracies, above all, they are of a
gradual and imperceptible nature. The citizens begin by giving up
some part of the constitution, and so with greater ease the
government change something else which is a little more important,
until they have undermined the whole fabric of the state. At Thurii
there was a law that