Sociology has long sought to find out how acting in a situation and observing that situation may differ and nevertheless belong to a single kind of social operation. George Spencer-Brown s Laws of Form (1969) provides one way to conceive of such an operation. The present book is the first to make sociological use of his mathematical calculus of form, which has been extensively applied to cybernetics, systems theory, cognitive science, and mathematics. Spencer-Brown s theory states that any action or communication is always an operation that makes a distinction. Not only does this operation take place, but it can be observed as indicating what it is interested in, and as leaving unmarked what it is not. Distinctions thereby entail a logic of inclusion and exclusion that is subject to social debate and conflict. In social situations there is no action that does not at the same time execute, maintain, or cross a distinction.