In recent years, there has been a growing interest in approaches that employ ontological models as theoretical tools for analyzing and improving conceptual modeling languages. In this paper we present a philosophically and cognitively well-founded formal ontology which has been developed with the special purpose of serving as a foundation for general conceptual modeling languages. Furthermore, we demonstrate how this foundational ontology named the Unified Foundational Ontology (UFO) has been used to evaluate and redesign the metamodel of the Unified Modeling Language (UML) for the purpose of conceptual modeling.