 How should we approach the completeness question? In the history of attempts to answer this question, people have tended to adopt one of two strategies. One appeals to (what philosophers call) modal notions, where a modal notion here means a notion connected to possibility and necessity, to what might or must be the case. Another appeals to non-modal notions, that is, notions distinct from ideas about possibility and necessity. There is a wide variety of such notions, though perhaps the most obvious one is identity in the logical sense, according to which if x is identical to y, then every property of x is a property of y. This notion has consequences for possibility and necessity, but it is not itself modal. Other non-modal notions include realization, or grounding, which we will consider below.