  How much of Ketonen's work stems from suggestions on the part of  Gentzen remains unknown, because no correspondence has been found.  Ketonen writes in the preface of his thesis that “Dr. G.  Gentzen of Göttingen directed me towards the problem area of  this work”. The thesis was Ketonen's only original work in  logic, saved from oblivion by a long review that Bernays wrote of it  for The Journal of Symbolic Logic in 1945.