As scarce as truth is, the supply has always been in excess of the demand. Josh Billings Desire versus Demand First, we need to clarify what we mean by demand. Demand means the quantity of any particular good that people will buy at a given price. Notice that demand is not how much of a good that people need or desire. Needs or desires may be keenly felt, but do not necessarily lead to actual purchases of goods or services. As the saying goes: “if wishes were horses, then beggars would ride.” People cannot buy what they cannot pay for. Therefore needs or desires are not effective demand. Effective demand means only the quantity is actually purchased. Also notice that the quantity that people will purchase is not fixed. It varies according to the price. So, in economics, the quantity actually purchased is never absolute, but always relative, depending on price. This is very important to remember. In casual conversation, people often mention "demand" as if it is one fixed amount. They say, for example, that demand for Lexus autos is high, or that demand for less popular makes of autos is low. This may be fine for casual conversation, but it is too vague and not specific enough for serious discussion of economic principles. The quantity demanded depends on the price because goods are scarce. If there was an unlimited supply of any particular good, everyone could have as much as they wanted, and there would be no-one willing to pay even a penny for that good.

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Economics Made Easy 2nd Edition For example, generally we have all the air that we need to breathe. So air is free. But that changes if air becomes scarce. For example, if air pollution becomes intense, we may need to purchase surgical masks in order to breathe. Similarly, if one has difficulty breathing because of heart or lung deficiencies, then one becomes willing to pay for oxygen. So it is scarcity