What is the law of demand?

The law of demand states that quantity purchased varies inversely with price. In other words, the higher the price, the lower the quantity demanded and vice versa. This occurs because of diminishing marginal utility. That is, consumers use the first units of an economic good they purchase to serve their most urgent needs first, and use each additional unit of the good to serve successively lower valued ends. An example of this is the satisfaction a user gets from eating pizza.