It is the job of the governing body to make those sorts of judgments, but not to rule by decree. Aristotle believes that the best attainable regime is polity. He uses the word polity in an unqualified sense when he is describing the best regime. Aristotle does the same thing when speaking of aristocracy, which, theoretically speaking is the best regime because it is the regime in which judges according to virtue and chooses its rulers on the basis of virtue. Aristotle recognizes that in its pure form such a standard is impossibly high, because it requires knowledge of what true virtue is, always as the ability to recognize true virtue in others.
Polity is a mixed regimes which combines elements of democracy and oligarchy.
Aristotle enumerates three possibilities for mixing regimes, but in the first two taking elements of legislation from each and taking the mean between arrangements the rich are still treated as rich and the poor are still treated as poor. The third, more preferable manner of mixing regimes takes some provisions from oligarchic law and some from democratic law. This method transcends the divisions between poor and rich by having a mixture of offices chosen by lot and by election, thereby integrated the oligarchic and democratic principles into a coherent whole. The middling element is basically what in modern terms would be considered the middle class. Aristotle praise of the middling element is logical, in that he believes virtue to be mean between two extremes of vice.