Book V is concerned with the sources of change within governments, as well as the destruction and preservation of the various forms of government.
As was said earlier in Book III, popular rule, which can be a form of tyranny, comes about under the belief that free people are also equal in every respect, and oligarchy arises with the belief that people are wholly unequal in every respect. Both claim to have justice but they ‘miss the mark’ if justice is taken in an unqualified sense. Both have a tendency to breed factional conflict. Sometimes people have qualms with the particular form of government, and other times they merely want control over their existing form of government. Faction is a result of “inequality,” but democracy is more stable and free of faction than oligarchy. Factions may arise by those seeking equality believing themselves to possess less than others, or those who aim at inequality have less or a more equal amount. People also form factions to avoid dishonor and fines, and out of fear and private gain over public benefit, along with “dissimilarity of races” all contribute to factions. Though he does not explicitly state it, Aristotle implies that resentment plays an important role in factionalism. Although factions start small, they can grow over time.
Democracies are susceptible to destruction by demagogues, some as generals, others as soothsayers. Kings start out as prominent men, while tyrants generally start out as self-seeking demagogues.
Oligarchies are susceptible to treating the people unjustly, thus facing a democratic backlash.
Aristocracies are susceptible to faction and destruction when when anyone but the best are honored.
All governments should necessarily be on guard against a habit of lawlessness – by keeping opposing groups in a balance of power, by limiting office terms, avoiding trickery by actually treating the people well, by not elevating any one particular group or citizen over another. Most of all, laws should prevent people from seeking profits. Democracies should avoid policies that impoverish the rich, and oligarchies should serve the needy.
Kings arise from honorable guardians. Their private lives have led to their downfalls, but the same factions can arise as in oligarchies and democracies. The longest lasting kingships willingly surrender parts of their authority. Some kings are overthrown when they become tyrannical, but also some kings have preserved their power by seeking to be respected rather than feared (note: Aristotle represents the classical teaching that is inverted in Machiavelli’s The Prince).
The various forms of government are not fixed: a tyranny can blend with others, and thus the degeneration of the city in the Republic does not match reality.