It is also important to distinguish at the outset the ontological commitments of a theory from its ideological commitments (following Quine 1951a). The ontological commitments of a theory are, roughly, what the theory says exists; a theory is ontologically committed to electrons, for example, if the truth of the theory requires that there be electrons. The ideological commitments of a theory are, roughly, those concepts, logical or non-logical, that are expressible within the theory.  Ontological commitment and ideological commitment are largely independent of one another. For example, a theory may be ontologically committed to there being Ks, even if the concept of being a K is not expressible in the theory.  In the other direction, a theory may be ideologically committed to relational concepts that add structure to the entities posited by the theory without adding new kinds of entity to the ontology.  There has been a trend in recent metaphysics to take ideological commitment to be as important—if not more important—than ontological commitment with respect to providing a criterion of theory choice, at least for fundamental theories (see especially Sider 2011: 12–15). In this article, however, only ontological commitment will be discussed.