Our ultimate goal is to provide a framework and a methodology which will allow users, and not only system developers, to construct complex systems by composing existing modules, or to add new modules to existing systems, in a “plug and play” manner. These modules and systems might be based on different logics; have different domain models; use different vocabularies and data structures; use different reasoning strategies; and have different interaction capabilities. This paper, which is a first small step towards our goal, makes two main contributions. First, it proposes a general architecture for a class of reasoning modules and systems called Open Mechanized Reasoning Systems(OMRSs). An OMRS has three components: a reasoning theory component which is the counterpart of the logical notion of formal system, a control component which consists of a set of inference strategies, and an interaction component which provides an OMRS with the capability of interacting with other systems, including OMRSs and human users. Second, it develops the theory underlying the reasoning theory component. This development is illustrated by an analysis of the Boyer-Moore system, NQTHM.