In this article the authors argue that an ontology based either upon ‘being’ or ‘doing’ will emerge, depending on which aspect of Peirce’s triadic model of the sign is emphasized: the object or the interpretant. Their argument is based on an analysis of two types of ontology used in object-oriented computer programming: namely, the class and the abstract data type. Among their findings is the observation that the programming ontology is determined by the object-types defined, rather than the inverse. Keywords: Object-oriented programming paradigm, class, abstract data type, Peirce’s sign model.