Locales are Not Pointless

The Kripke-Joyal semantics is used to interpret the fragment of intuitionistic logic containing ^;! and 8 in the category of locales. An axiomatic theory is developed that can be interpreted soundly in two ways, using either lower or upper powerlocales, so that pairs of separate results can be proved as single formal theorems. Openness and properness of maps between locales are characterized by descriptions using the logic, and it is proved that a locale is open ii its lower powerlocale has a greatest point. The entire account is constructive and holds for locales over any topos.