Price, in many cases, is likely to be the most fundamental determinant of demand, since it's often the first thing that people think about when deciding how much of an item to buy. The vast majority of goods and services obey what economists call the law of demand
- that, all else being equal, the quantity demanded of an item decreases when the price increases and vice versa. (There are some exceptions to this rule, but they are few and far between.)
As we will see, this is why the demand curve slopes downwards.