[...] there are situations in which they don't seem to work all that well--and not merely relative to the perfection some assume they should attain, but even relative to suitably diminished "real-world" expectations. Economists call these "market failures," and perhaps the most commonly noted is the case of so-called "public goods." In political economy, public goods are not just things that are good for the public, but goods or services that it helps everyone to have, but for which there is insufficient incentive for capitalist investment. [...] even if an entrepreneur could come up with a way of "cleaning" the air, there is no way he or she could make a return on their investment because it would be impossible to prevent people from breathing cleaned air for free. Clean air, at least so far, is what economists call "nonexcludable" and "nonrival"--you may be able to provide it for a price, but it is impossible to prevent someone from using it at no cost (nonexcludability), and no matter how much one person uses, it does not diminish available supplies (non-rivalry). In the case of clean air, then, there is no market incentive to provide it: you can't exclude those who don't pay from using it, and no matter who or how many uses it, there is always enough left for others. In other words, you can't control its distribution via contract and you can't make it scarce enough to merit a price.