Marginalism is a theory of economics that attempts to explain the discrepancy in the value of goods and services by reference to their secondary, or marginal, utility. It states that the reason why the price of diamonds is higher than that of water, for example, owes to the greater additional satisfaction of the diamonds over the water. Thus, while the water has greater total utility, the diamond has greater marginal utility.

Although the central concept of marginalism is that of marginal utility, marginalists, following the lead of Alfred Marshall, drew upon the idea of marginal physical productivity in explanation of cost. The neoclassical tradition that emerged from British marginalism abandoned the concept of utility and gave marginal rates of substitution a more fundamental role in analysis. Marginalism is an integral part of mainstream economic theory.

Source: Wikipedia

Marginalism, one of the methodological principles of bourgeois political economy, came into existence when the shift from free competition to all-powerful monopolies placed before economists tasks which could not be implemented by a strict reliance on the subjectivistic understanding of economic processes. Marginalism, therefore, deviates from such subjectivism by viewing economics as the interaction of individual economies.
(F) Fuzzy exceptional problems