 Truth values thus prove to be an extremely effective instrument for a logical and semantical analysis of   language.[1]  Moreover, Frege provides truth values (as proper referents of sentences) not merely with a pragmatical motivation but also with a strong theoretical justification. The idea of such justification, that can be found in Frege 1892, employs the principle of substitutivity of co-referential terms, according to which the reference of a complex singular term must remain unchanged when any of its sub-terms is replaced by an expression having the same reference. This is actually just an instance of the compositionality principle mentioned above. If sentences are treated as a kind of singular terms which must have designations, then assuming the principle of substitutivity one “almost inevitably” (as Kurt Gödel (1944: 129) explains) is forced to recognize truth values as the most suitable entities for such designations. Accordingly, Frege asks: