 In words, \(\cstit_i \phi\) is true at an index \((h,m)\) if \(\phi\) is true with respect to \(h'\) and \(m\), for all histories \(h'\) in the same choice cell at \(m\) as \(h\); this is called the positive condition. The truth-condition for \(\dstit_i \phi\) is more exacting; not only the positive condition has to be satisfied but also what is called the negative condition: there must be some history through \(m\) such that \(\phi\) fails to be true with respect to that history and \(m\).