Editing Marginalism (section)

=== Demand ===
Demand curves are explained by marginalism in terms of marginal rates of substitution.

At any given price, a prospective buyer has some marginal rate of substitution of money for the good or service in question.  Given the "law" of diminishing marginal utility, or otherwise given convex indifference curves, the rates are such that the willingness to forgo money for the good or service decreases as the buyer would have ever more of the good or service and ever less money. Hence, any given buyer has a demand schedule that generally decreases in response to price (at least until quantity demanded reaches zero). The aggregate quantity demanded by all buyers is, at any given price, just the sum of the quantities demanded by individual buyers, so it too decreases as price increases.