Abstract :
Not all predicate calculus WFF correspond to meaningful questions. In order to avoid this problem, different authors have defined syntactically the WFF classes which are known to be significative. These restrictions are generally more severe than is necessary, and we have defined a much wider class of WFF : the evaluable formula. We prove that these WFF have a clearly defined sense. Moreover, we can easily test a formula to see if it is evaluable. Finally, we show how it is possible to deduce from a formula the conditions which have to be ful-filled by the predicate argument validity domains in order to obtain answers which are not an empty set. We can thus reject questions which have a defined sens but which, in the context of a clearly determined application, cannot have an answer.
