The law of demand isn't really a law, like the law of gravity. There are exceptions to this law, and we must also specify that only price (and nothing else) changes, in order for the law to be correct. Nevertheless, it is worth stating:

   The Law of Demand: If nothing else changes that affects demand, an increase in price will lead to a decrease in desired quantity demanded, and a decrease in price will lead to an increase in desired quantity demanded.

   It's kind of a weenie law, isn't it? That first clause, "If nothing else changes...." is sometimes written "All else being equal," or ceteris paribus. Can you imagine how you'd feel if the law of gravity didn't work all the time (lighter maybe)?

   However, it turns out that this wimp clause is vitally important. Plenty of things, other than price, affect demand; and if one or more of them is changing at the same time that the price is changing, we can't be sure which effect we're seeing. So, we need this clause.

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