Functionalism is the original and still dominant discipline of thought in the social sciences. As a construct of two forms of scientific investigation: the scientific approach and viewing the individual as a part of a social organism or social whole, the scientific method considers society as an objectively observable and "real" entity that is suitable for methods and philosophies that guide examination and study of the physical world.
The concepts of objectivity, or of eliminating personal bias, along with personal disinterest, which allows identification of the laws which govern social behavior are the core of the functionalist approach. Scientific and quantitative approaches were furthered by Emile Durkheim, who saw the benefits of hard scientific approaches and quantitative approaches in reducing observer bias and interference with the subject matter.
The analogical approach involves viewing society as a real entity. Society is viewed as an organism which consists of individual entities which perform specific functions which keep the society in existence. Religion is an individual entity that contributes to the existence of society, but which is also affected by the society. In other words, under the functionalist perspective, the society, as an organism "survives" through the functions, which operate in the way that the organs of a biological entity do in order to insure it's survival.
Social equilibrium is a condition that is upset, then restored as external and internal challenges to important norms and values, occur, and as consensus between the individual "organs" of a society reach consensus. The analogy to the biological organism or physical entity is that equilibrium is upset when the biological organism suffers poisoning, for example. In a normal return to equilibrium, the organs process the poison in ways that restore equilibrium in the biological entities system and the entity continues to live. Sociologists attempt to identify the conditions which cause disequilibrium, and the mechanisms by which equilibrium is restored.
Some functionalists use "systems theory", as developed by Talcott Parsons during the 1950s and 1960s, at the time when systems theory was becoming popular in the biological and computer sciences. Parsons viewed social entities as constructs of systems that existed within systems, within systems.
Some functionalists focused on social stratification, inspired by geological understandings of how rocks became stratified, or layered over time. In looking at the pressures and exposures of society on the individual's place or role within the society, studies of caste, class and race were enhanced by looking at social strata much as geologists look at rock strata.
While specific philosophies that favored the hard scientific approach that led to Parson's system theory, or looking at individual humans as biological "cells" in a larger organism have been refined or replaced by more sophisticated analytical tools, the functionalist method, structure, and discipline of analytical thought is still the overriding discipline of thought for today's Sociologist.