Basic Truth Maintenance is a common feature, available in many production rule systems, but it one that is not generally well understood. This talk will start by introducing the mechanics of rule engines and how that is extended for the common TMS implementation. It will discuss the limitations of these systems and introduce Justification based Truth Maintenance (JTMS) as a way to add contradictions, to trigger retractions. This will lead onto Defeasible Logic, which while sounding complex, facilitates the resolving of conflicting rules of premisses and contradictions, in a way that follows typical argumentation theory. Finally we will demonstrate how the core of this can be abstracted to allow pluggable beliefs, so that JTMS and Defeasible can be swapped in and out, along other systems such as Bayesian Belief Systems.