We will use the utility theory to explain consumer demand and to understand the nature of demand curves. For this purpose, we need to know the condition under which I, as a consumer, am most satisfied with my market basket of consumption goods. We say that a consumer attempts to maximize his or her utility, which means that the consumer chooses the most preferred of goods from what is available.
Can we see what a rule for such an optimal decision would be? Certainly I would not expect that the last egg I am buying bring exactly the same marginal utility as the last pair of shoes I am buying, for shoes cost much more per unit than eggs. A more sensible rule would be: If good A costs twice as much as good B, then buy good A only when its marginal utility is at least twice as great as good B’s marginal utility.
This leads to the equimarginal principle that I should arrange my consumption so that every single good is bringing me the same marginal utility per dollar of expenditure. In such a situation, I am attaining maximum satisfaction or utility from my purchases. This is clear concept of equimarginal principle.