There only can the government ever be stable where the middle class exceeds one or both of the others, and in that case there will be no fear that the rich will unite with the poor against the rulers.
 Aristotle, Politics (4th century BC). copy citation

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Author Aristotle
Source Politics
Topic government fear
Date 4th century BC
Language English
Reference
Note Translated by Benjamin Jowett
Weblink http://classics.mit.edu/Aristotle/politics.html

Context

“The legislator should always include the middle class in his government; if he makes his laws oligarchical, to the middle class let him look; if he makes them democratical, he should equally by his laws try to attach this class to the state. There only can the government ever be stable where the middle class exceeds one or both of the others, and in that case there will be no fear that the rich will unite with the poor against the rulers. For neither of them will ever be willing to serve the other, and if they look for some form of government more suitable to both, they will find none better than this, for the rich and the poor will never consent to rule in turn, because they mistrust one another.” source