sentences:  There are many variations of realizability; exactly which class of formulas is studied and which objects are realizers differ from one variation to another.Realizability can be seen as a formalization of the BHK interpretation of intuitionistic logic; in realizability the notion of "proof" (which is left undefined in the BHK interpretation) is replaced with a formal notion of "realizer".Most variants of realizability begin with a theorem that any statement that is provable in the formal system being studied is realizable.