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Book VIII–IX: Plato's five regimes

In Books VIII–IX stand Plato's criticism of the forms of government. Plato categorized governments into five types of regimes: aristocracy, timocracy, oligarchy, democracy, and tyranny.

The starting point is an imagined, alternate aristocracy (ruled by a philosopher-king); a just government ruled by a philosopher king, dominated by the wisdom-loving element. Aristocracy degenerates into timocracy when, due to miscalculation on the part of its governing class, the next generation includes persons of an inferior nature, inclined not just to cultivating virtues but also producing wealth. In a timocracy, governors will apply great effort in gymnastics and the arts of war, as well as the virtue that pertains to them, that of courage. As the emphasis on honor is compromised by wealth accumulation, it is replaced by oligarchy. The oligarchic government is dominated by the desiring element, in which the rich are the ruling class. Oligarchs do, however, value at least one virtue, that of temperance and moderation—not out of an ethical principle or spiritual concern, but because by dominating wasteful tendencies they succeed in accumulating money.

As this socioeconomic divide grows, so do tensions between social classes. From the conflicts arising out of such tensions, the poor majority overthrow the wealthy minority, and democracy replaces the oligarchy preceding it. In democracy, the lower class grows bigger and bigger. The populism of the democratic government leads to mob rule, fueled by fear of oligarchy, which a clever demagogue can exploit to take power and establish tyranny where no one has discipline and society exists in chaos. In a tyrannical government, the city is enslaved to the tyrant, who uses his guards to remove the best social elements and individuals from the city to retain power (since they pose a threat), while leaving the worst. He will also provoke warfare to consolidate his position as leader. In this way, tyranny is the most unjust regime of all.

In parallel to this, Socrates considers the individual or soul that corresponds to each of these regimes. He describes how an aristocrat may become weak or detached from political and material affluence, and how his son will respond to this by becoming overly ambitious.The timocrat in turn may be defeated by the courts or vested interests; his son responds by accumulating wealth in order to gain power in society and defend himself against the same predicament, thereby becoming an oligarch. The oligarch's son will grow up with wealth without having to practice thrift or stinginess, and will be tempted and overwhelmed by his desires, so that he becomes democratic, valuing freedom above all. The democratic man is torn between tyrannical passions and oligarchic discipline, and ends up in the middle ground: valuing all desires, both good and bad. The tyrant will be tempted in the same way as the democrat, but without an upbringing in discipline or moderation to restrain him. Therefore, his most base desires and wildest passions overwhelm him, and he becomes driven by lust, using force and fraud to take whatever he wants. The tyrant is both a slave to his lusts, and a master to whomever he can enslave. Socrates points out the human tendency to be corrupted by power leads down the road to timocracy, oligarchy, democracy and tyranny. From this, he concludes that ruling should be left to philosophers, who are the most just and therefore least susceptible to corruption. This "good city" is depicted as being governed by philosopher-kings; disinterested persons who rule not for their personal enjoyment but for the good of the city-state (polis). The philosophers have seen the "Forms" and therefore know what is good. They understand the corrupting effect of greed and own no property and receive no salary. The philosopher-kings also live in sober communism, eating and sleeping together among themselves.