 Admittedly, those who defend a modal-existential analysis of ontological dependence could insist that it applies only to contingent objects (cf. Simons 1987: 295). Simons makes this type of qualification by focusing on concrete entities, hence excluding necessary existents by definition; he also excludes self-dependence. Simons calls the resulting notion of dependence weak rigid dependence, but a stronger notion, strong rigid dependence (Simons 1987: 303) is also defined—the latter is a special case of the former. One example of weak rigid dependence as defined by Simons would be a particular water molecule depending for its existence on a particular oxygen atom. In the case of strong rigid dependence, the dependent object cannot be a proper part of the object it depends upon. So object x is strongly rigidly dependent on object y if x depends for its existence on y and y is not a proper part of x. One example of strong rigid dependence defined thus would be a trope (or mode) depending for its existence on a substance. In addition to these rigid notions, Simons defines corresponding notions of (weak and strong) generic dependence.