Public Goods vs Private Goods

Economists sort goods using two questions. First, is the good rivalrous? In other words, when one person uses it, does that reduce how much is left for others? Second, is the good excludable? Can a seller stop people who do not pay from using it? An apple is rivalrous and excludable: if you eat it, no one else can, and the store can refuse to give it to you unless you pay. That makes apples a private good.

A public good is the opposite. It is non-rivalrous and non-excludable. A lighthouse is the classic example. One ship using its light does not dim it for another. And once the light is shining, every ship in range benefits, whether they paid or not. Clean air, basic scientific research, mosquito control, and national defense all look the same way. Markets struggle to provide pure public goods because no private company can collect payment from everyone who benefits, and people have an incentive to free-ride: enjoy the benefit without paying.

Many real-world goods sit between the extremes. A city park is mostly non-rivalrous on a quiet day, but rivalrous if it gets crowded. A toll road is excludable but not very rivalrous if traffic is light. These mixed cases, sometimes called common-pool resources or club goods, are where some of the toughest policy fights happen, including fishing rights, internet access, and vaccines.

Knowing whether something is a public or private good will not give you a final answer about who should provide it. Plenty of goods could be public or private depending on choices a society makes. But the framework gives you a much sharper way to ask: why is the government in this market at all, and is there a better way to structure it?