Classical economics
Prices naturally adjust to having more or less of something: supply and demand. If there is too much of something (more supply), prices go down so that people have more reason to buy it. If a lot of people want something (high demand), there may not be enough of it (scarcity); there will be a competition among the people who want to buy it, and some people will agree to pay more to get what they want, and prices go up.