The law of demand was given by Prof. Alfred Marshall in his book "Principles of economics" published in 1890. The law states the inverse relationship between price and quantity demanded .
There are certain assumptions given by Marshall to ensure that the law holds true and is observed to be working.
The assumption are:
1) Constant level of consumers' income
2)Lassez Faire Policy ( No government intervention in business activities)
3) Constant level of population.
4) No change in the habits , taste , fashions and so on.
5)No speculation about future prices by the consumers.
6)Prices of complementary and substitute goods to remain constant.
7)No change in state of technology.
8)No change in climatic conditions.
9) Constant cultural environment ( Customs and traditional festives)
10) Constant level of taxation and No change in government policies).