The law of substitution is also known as the law of equi-marginal utility or the law of maximum satisfaction. This law was first developed by H.H Gossen. Therefore, this law is also known as second law of Gossen. Prof. Marshall has developed and given the present shape of this law.
This law states that in order to get maximum satisfaction, a consumer should spend his limited income on different commodities in such a way that the last dollar spent on each commodity yield him equal marginal utility.
The law of substitution is also known as " The Law Of Maximum Satisfaction" because the consumer can maximize his/her satisfaction by spending income in accordance with this law. It is called " The Law Of Substitution" because the consumer will go on substituting one commodity with higher marginal utility for another commodity with lower marginal utility till the marginal utility of each commodity is equal. Suppose, there are two commodities X and Y on which a consumer has to spend a given income. If he finds that the marginal utility of commodity X is higher than the marginal utility of commodity Y, he will substitute the former for the latter till their marginal utilities are equalized.
