Extremes become unreasonable, while the mean is reasonable. Finally the middle class can encourage friendship and cooperation. Politics needs such friendship and unanimity -rule of equals and peers. These factors upset political stability, and ultimately do away with political freedom as well. That is, they tend to tyranny.
Middle class can temper the extremism.
We must next turn to the dynamic whereby the oligarchy and democracy verge down toward tyranny or up toward polity. He would leave that to the ethics. He does dwell on their political justice. Thus the Oligarch and Democrat take one side of justice and push it to an extreme. Their idea of justice is correct as far as it goes - but it is partial and cannot go all the way without incurring in justice.