John Bates Clark was an American economist who lived in the late 19th and early 20th centuries. He is considered one of the founders of the neoclassical school of economics and is best known for his work on marginal productivity theory.

One of Clark's most influential works is his book "The Essentials of Economic Theory" which was first published in 1907. The book provides an introduction to economic theory and covers a wide range of topics, including production, consumption, distribution, and exchange.

Clark's approach to economic theory was highly mathematical and he believed that economic analysis should be based on the principles of marginalism. He argued that the value of a good or service is determined by the marginal utility it provides to consumers and the marginal cost of producing it.

Clark also developed the theory of marginal productivity, which suggests that the value of a worker's contribution to the production process is equal to the marginal product of labor. This theory has been highly influential in the development of modern labor economics.

Overall, "The Essentials of Economic Theory" is an important work that has had a significant impact on the development of economic thought. Clark's emphasis on the principles of marginalism and his development of the theory of marginal productivity have helped to shape the way that economists think about markets and the role of individuals in the production process.