(iii) The law assumes that utility can be measured. But, it is a subjective mental phenomenon, which cannot be measured or compared on a cardinal scale. Further, constancy of marginal utility of money is open to question.
(iv) The law assumes that the fashion, tastes, habits, customs and income of people remain constant. But, in real life these things, seldom remain constant. Moreover, many people are bound by customs, traditions and fashions, which also change over time. Hence, consumers often spend irrationally on relatively less useful goods or goods yielding low utilities.
(v) There is problem associated with the durable goods, which are used over a long period of time. Expenditure on such goods is increased during one period, but utility derived from it is spread over many time periods. It becomes difficult for a consumer to equate the MU of services rendered by such commodities in each period.
(vi) In real life, consumer hardly tries to compare marginal utilities of different commodities, when the amount of expenditure involved is very small. The law is, thus, ignored by the consumer in his behaviour pattern and he often buys relatively inessential goods.
(vii) The law is not applicable in the case of complementary goods, which cannot be substituted for each other.
In spite of the limitations described above, the law does not lose its fundamental validity.