A good or commodity in economics is any object or service that increases utility, directly or indirectly, not to be confused with good in a moral or ethical sense (see Utilitarianism and consequentialist ethical theory). A good that cannot be used by consumers directly, such as an office building or capital equipment, can also be referred to as a good as an indirect source of utility through resale value or as a source of income. A 'good' in economic usage does not imply moral acceptance or even legality.
If an object or service is sold for a positive price, then it is a good since the purchaser considers the utility of the object or service more valuable than the money. Some things are useful but not scarce such as air and are referred to as free goods.
A good is an object whose consumption increases the utility of the consumer, for which the quantity demanded exceeds the quantity supplied at zero price. Goods are usually modeled as having decreasing marginal utility. The first car an individual purchases is very valuable; the fourth is much less useful. Thus, in these and similar goods, the marginal utility of additional units approaches zero as the quantity consumed increases. Assuming that one cannot re-sell it, there is a point at which a consumer would decline to purchase an additional car, even at a price very near zero. This is the consumer's satiation point.
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