 This approach evidently leaves the question of how to understand the idea of modal force (of a proposition’s truth being very unshakable). Some authors have attempted to explain this notion in counterfactual terms (see Lewis 1973a, §2.5; Lewis 1973b, §2.1; McFetridge 1990, 150ff.; Lange 1999, 2004, 2005; Williamson 2005, 2008; Hill 2006; Kment 2006a; cp. Jackson 1998, Chalmers 2002a): the necessary truths are distinguished from the contingent ones by the fact that they are not only true as things actually are, but that they would still have been true if things had been different in various ways. To capture this idea more precisely, Lange (2005) introduces the concept of ‘stability’: a deductively closed set \(S\) of truths is stable just in case, for any claim \(P\) in \(S\) and any claim \(Q\) consistent with \(S\), it is true in any context to say that it would still have been the case that \(P\) if it had been the case that \(Q\). The different forms of necessity have in common that their extensions are stable sets.