Talcott Edgar Frederick Parsons was an American sociologist who, as the principal exponent of what is known as structural functionalism, exerted a major influence over social theory in the middle part of the twentieth century. His work continues to be an important influence for German sociology. Primarily concerned with explaining the stability and order of social life, Parsons draws on ideas developed in cybernetics, in particular on what is known as systems theory. A system may be understood as a stable and organized structure that exists within an environment. Parsons argues that society, understood as a social system, must satisfy four functions if it is to remain stable.