In the preceding chapters I have attempted to provide a criterion for the meaningfulness of informative expressions. An expression conveys information if it gives rise to suggestions about possible future experiences in certain possible circumstances. An expression of this sort is such that it may be found to mean a probably true proposition or it may be found to mean a probably false one. So that we have been discussing sentences that mean contingent propositions. And the test for truth lies always in the immediate experiences of the person who tries to verify the proposition. So that we have been discussing signs for a posteriori propositions. There has been discovered nothing in the signs themselves that could indicate their truth or falsity.1 But a theory of Communication would not be complete without a discussion of an alleged use of language to convey propositions which would commonly be described as of a different sort, that is a priori and necessary.
Consider, as examples of so-called Necessary a priori propositions:
All white swans are white
All white swans are birds
No white swans are black.
It has often been maintained that these propositions, once all their terms are clearly understood, can be seen to be true without making any experiments or observations at all;____________________