 The above explanation of theoreticity may be felt unsatisfactory as it determines the property of being theoretical only via negation of the property of being observable (Putnam 1962). This explanation does not indicate any specific connection between the semantics of theoretical terms and corresponding scientific theories. There is, however, also a direct characterization of theoreticity that complements the criterion of non-observability: an expression is theoretical if and only if its meaning is determined through the axioms of a scientific theory. This explanation rests on what has come to be referred to as the contextual theory of meaning, which says that the meaning of a scientific term depends, in some way or other, on how this term is incorporated into a scientific theory.