The Law of Demand:
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Other factors remaining constant (ceteris paribus) there is an inverse relationship between the price of a good and its demand. Demand varies inversely with price, not necessarily proportionately. If the price falls, demand will extend and vice-versa. Demand thus is a function of price. i.e. it varies with price. It can be expressed as:
D = f (P)
Where,
D = Demand
P = Price



The ceteris paribus assumption:
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Ceteris paribus means all other factors remaining the constant. Many factors can be said to affect demand. Economists assume all factors are held constant except one–the price of the product itself. Thus, the assumptions of the law are:

1. No change in the consumer`s income.
2. No change in consumer`s tastes and preferences.
3. No changes in the prices of inter-related goods.
4. No new substitutes for the goods have been discovered.
5. The use of a commodity does not confer distinctions.