“The best constitution is one controlled by a numerous middle class which stands between the rich and the poor. For those who possess the goods of fortune in moderation find it “easiest to obey the rule of reason” (Politics IV.11.1295b4–6). They are accordingly less apt than the rich or poor to act unjustly toward their fellow citizens.
A constitution based on the middle class is the mean between the extremes of oligarchy (rule by the rich) and democracy (rule by the poor). “That the middle [constitution] is best is evident, for it is the freest from faction: where the middle class is numerous, there least occur factions and divisions among citizens” (IV.11.1296a7–9). The middle constitution is therefore both more stable and more just than oligarchy and democracy.
Source – stanford.edu