The aim which he sets before oligarchs or democracies is not the good life, but simple stability or permanence of the existing constitution.
With this spirit of realism which pervades Books IV., V., and VI.
the idealism of Books I., II., VII., and VIII.

is never reconciled.
Aristotle is content to call existing constitutions perversions of the true form.

But we cannot read the Politics without recognising and profiting from the insight into the nature of the state which is revealed throughout.

Aristotle's failure does not lie in this, that he is both idealist and realist, but that he keeps these two tendencies too far apart.

He thinks too much of his ideal state, as something to be reached once for all by knowledge, as a fixed type to which actual states approximate or from which they are perversions.