To choose their actions, reasoning programs must be able to draw conclusions from limited information and subsequently revise their beliefs when discoveries invalidate previous assumptions. A truth maintenance system is a problem solver subsystem for performing these functions by recording and maintaining the reasons for program beliefs. These recorded reasons are useful in constructing explanations of program actions in "responsible" programs, and in guiding the course of action of a problem solver. This paper describes the structure of a truth maintenance system, methods for encoding control structures in patterns of reasons for beliefs, and the method of dependency-directed backtracking.
