The last pair is republic and democracy. A republic is rule by representation; a democracy is literally rule by the mob, through direct decision making by the populace. Democracies are a degenerate form of government because they are inevitably susceptible to falling under the control of a demagogue, a charismatic leader who can sway people to her or his own ends rather than to the common good. Republics are buffered from this danger by the representatives who run the government, yet