 Formula (6) is equivalent to \(\phi \rightarrow \bF\phi\). The latter formula says that if \(\phi\) is the case then \(\phi\) will be the case, which is intuitively incorrect. Suppose that \(\prec\) is irreflexive; this assumption makes sense since it means that no time point is earlier than itself. But under this assumption, Principle (6) is obviously wrong. For extensions of temporal logic and more examples concerning the correspondence between properties of models and the validity or invalidity of formulas see Gamut 1991: chapter 2.