Frederic Bastiat and Subjective Marginal Utility

"In the 1870s economics took a radical turn in what is known as the “marginal revolution.” Whereas the classical economists, beginning with Adam Smith, cleaved use-value from exchange-value and thought in terms of the total utility and total supplies of goods, Carl Menger, William Stanley Jevons, and Leon Walras realized that people act at the margin. They never choose between all the water and all the diamonds; rather, they act to acquire an additional unit of water or that particular diamond."