Plato claims that justice in a republic was the same virtue as justice in a human being, and that the parts of a human soul match those in a state and stand in the same just or unjust relationships to one another. A monomaniac runs his life as a tyrannical state runs itself.
Plots (Leonard Hastings Schoff Lectures)
Robert Belknap
In 19th-century psychiatry, monomania (from Greek monos, “one”, and mania, meaning “madness” or “frenzy”) was a form of partial insanity conceived as single psychological obsession in an otherwise sound mind.
https://en.wikipedia.org/wiki/Monomania