Our civilization is a complex system of individuals interacting with the environment. Societal dynamics develops via human actions, which are governed by decisions. The action of individuals is always a choice among the possibilities recognized as allowed by the circumstances. The choice always implies a decision. The theory of human decisions will be discussed with an emphasis on the difference between simple system and complex system approaches. The basic criterion for decisions is that the best is selected, but which is the best one? Founders and best proponents of utilitarianism, Jeremy Bentham (1748-1832) and John Stuart Mill (1806-1873) started with the postulate known as the “greatest happiness principle”. It can be viewed as the governing law of human actions. In the first part we summarize the way in which the greatest happiness principle did appear in the history. In the second part we outline a new approach to axiomatic economics, with the greatest happiness principle as the basic axiom. The result is a new dynamic microeconomics where the maximization rule of rational-choice decisions is replaced by a force law. The resulting theory is consistent with both physics and economics, especially the resource and environmental branches.