The purpose of public policy is to ensure a stable, sustainable progress.
Stability: Events such as war, economic bubbles & depressions, and natural disasters have a destabilizing effect on society. They can cause widespread death and destitution, both morally untenable. Public policy should seek to minimize instability by promoting peace and controlled economic growth, and providing relief in times of crisis.
Sustainability: A society that does not plan for the long-term cannot endure. Limited natural resources and a environmental damage should be accounted for appropriately. Specifically, the cost of pollution and depletion of limited resources should be factored into the price of goods and service. When the market fails to do this, it is the governments responsibility to do so.
Progress: Technological progress, rising standard of living, and the reduction of human suffering are the three key indicators of progress. Public policy should incentivize each.