The universe is all about balance. The forces of light and darkness are meant to keep a check on one another. If one becomes too powerful and starts overrunning the other, that balance will be upset. For the tyranny of virtue is as unbearable as the stranglehold of vice.
Russell observes that "the merits of democracy are negative: it does not ensure good government, but it prevents certain evils," such as the evil of a small group of individuals achieving a secure monopoly on political power. The chief peril for the politician, Russell insists, is love of power. And politicians can easily yield to the love of power on the pretense that they are pursuing some absolute good.
It was no accident that the beau ideal of his (John Adams') political philosophy was balance, since he projected onto the world the conflicting passions he felt inside himself and regarded government as the balancing mechanism that prevented those factions and furies from spending out of control.