The book introduces the principle of Least Action of classical mechanics into Economics. The book mathematically proves the laws of the free market, supply and demand and interest rate. Students, academics and public can learn how the principle of least action as the unity of economy allows to prove that money and all other products and assets are indispensable and mutually complimentary equivalents of each other that contribute to total utility in the form of constant money as the result of all human purposeful actions. Such unity and complementarity of all economic elements constitutes economic reality. This innovative approach to Economics allows to prove of the law of diminishing marginal utility, the production learning function, GDP model, theory of money as Universal equivalent. The uncertainty principle and subjective value. Growth of information in the form of interest rate as growth of entropy. Explanation of savings theory, the Prospect Theory, Big Data, Game Theory and other topics.