Source : Webster's Revised Unabridged Dictionary (1913)

Cogency \Co"gen*cy\, n. [See {Cogent}.]
   The quality of being cogent; power of compelling conviction;
   conclusiveness; force.

         An antecedent argument of extreme cogency. --J. H.
                                                  Newman.

Source : WordNet®

cogency
     n 1: persuasive relevance
     2: the quality of being logically valid [syn: {validity}, {rigor},
         {rigour}]