Structural-functionalism is a compound of two broad social theories, or theory sets -- "structural" and "functional" theories -- which attempt to show how society is structured (or organized) and how its identified parts relate to one another in actual (real-life) social interactions and processes. Structural-functionalism are two sides of the same coin. You cannot have structures without functions, and no functions without the structures whose consequences and reciprocal rationalizations they happen to be and in fact are.
Structure refers to the inter-related, and inter-relatedness of, parts constitutive of a given type of social organization, which are required for its effective and sustained operation. Indeed, a discrete social organization (or system), which we may give a more specific label (such as society, or family or community), is the "social fact" that embodies the more abstract idea of structure, and structure is what enables us to see, by way of metaphors and similes, the shape and form of a generalized or particular kind of social organization. Social structure and social organization are thus, in effect, also one and the same idea. But that is still only one half of the story; for it is important to add that, in this sameness, structure represents what, and only what, we may call the statics of social organization.
Social organization has another side -- the functional side. Since it is to be understood that social organization is the product and reflection of social inter-action, its components must, ipso facto, be understood to relate to each other in motion -- some kind of motion, even if not necessarily perpetual motion. Hence the idea that the flip side -- a necessary side -- of social statics is social dynamics.
The functional side is the dynamic side of the social organization. However, it is functional not simply by virtue of the motion we observe in the moving parts, but of something more fundamental to sociologists. It is functional in the sense of the consequences of the motion(s) that may at first attract attention. Both Durkheim and Merton saw "function" in those terms; the consequences being the "solidarity" which the parts generate for and among themselves, and the adaptive capacity which the parts, in their motion and solidarity, confer upon the whole.
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