public good

Think of a lighthouse on a rocky coast. Once its beam is lit, every passing ship sees it — the keeper can't shine it on one captain and hide it from another, and one ship using the light doesn't dim it for the next. A firework display, national defence, clean air, and a streetlight all share this odd quality: once provided, they are there for everyone, all at once. Economists call such things public goods, and they are the textbook case of something markets struggle to supply.

A pure public good has two defining traits. It is non-rival: one person's use does not reduce the amount available to others (my enjoying the fireworks leaves just as many for you). And it is non-excludable: there is no practical way to stop people who don't pay from enjoying it (you can't fence off the night sky). These differ from ordinary 'private goods' like an apple, which is both rival (I eat it, you can't) and excludable (no payment, no apple). Many things sit in between — a toll road is excludable but largely non-rival until it jams.

The trouble is that non-excludability invites the free-rider problem: why pay for something you'll get anyway? If everyone reasons this way, no private firm can collect enough revenue to provide the good, so the market under-supplies it or doesn't supply it at all — even though everyone wants it. This is why governments, charities, or collective agreements typically fund defence, basic research, public health, and clean air. The honest caveat: 'public good' is a technical label about excludability and rivalry, not a synonym for 'good for the public' — and clever pricing, technology, or community norms sometimes let markets provide them after all.