Short answers, pulled from the story.
Antoine Augustin Cournot published the book titled Theorie mathematique de la richesse sociale in 1838. This work introduced the idea that economic forces could be balanced like physical objects in nature.
Food was exported to England even as people starved because the equilibrium price exceeded what farmers could afford. The market cleared technically yet human suffering persisted while prices remained above affordability thresholds.
Huw Dixon outlined three fundamental properties that define consistent market outcomes in his 1990 analysis Surfing Economics. Property one states that agent behavior must remain consistent with established rules, property two requires no participant has an incentive to alter actions, and property three demands stability through a dynamic process returning the system to balance after disruption.
Monopolies often fail the second test because consumers want more than is available at the profit-maximizing quantity where marginal revenue equals marginal cost. Competitive equilibria satisfy all three properties including consistency and lack of incentive to change which monopolies frequently do not meet.
At $5.00 both sides equal 12,000 units creating a stable point in the numerical example. If the current price drops to $3.00 excess demand reaches 8,000 units generating upward pressure while rising to $8.00 produces an oversupply of 12,000 units pushing prices downward.