The law is based on certain assumptions which are as follows:
(1) Commodity must be Homogeneous:
The unit of the commodity consumed must be similar in quality, quantity, size, design and packing. If the first chocolate is smaller and the second chocolate is bigger the consumer gets more satisfaction. If the first chocolate is sweet and the second chocolate is sweeter the law will not operate. Therefore, the units of commodity consumed must be homogeneous.
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(2) Consumption must be continuous:
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If a person takes one glass of sarabat in the morning and other glass in the evening the law will not operate. There should be no time lag between the consumption of the units of the commodity.
(3) The consumer must be a normal one:
The law assumes that the consumer should be a normal one because if he behaves abnormally during the period of consumption, then the utility may be higher.
(4) Taste, Fashion and income of the consumer should remain the same during the period of consumption:
A change in the fashion, habit and income will not allow the law to operate. For example, if a particular commodity which was out of fashion yesterday if becomes fashionable today then marginal utility will increase.
Habit also interferes with the law. If a non- smoker becomes a smoker the marginal utility of the cigarette will increase for him. Similarly, if the income of a person increases the law will not operate.
(5) Sufficient quantity to be consumed:
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If the units of the commodity consumed are not sufficient in number the law will not operate. To a thirsty man if spoonful of water is given continuously the law will not operate.
The more he takes the more becomes marginal-utility. So he must be given glassful of water and successive glasses will give him less and less of satisfaction.
(6) Cardinal measurement of utility:
The law is based on the cardinal measurement of utility which assumes that the utility of a particular good is expressed numerically.