Eusebius is apparently thinking of the preface to his work contained in Bk. I. chap. 1, but there he makes no such promise as he refers to here. He speaks only of his general purpose to mention those men who preached the divine word either orally or in writing. In Bk. III. chap. 3, however, he distinctly promises to do what he here speaks of doing, and perhaps remembered only that he had made such a promise without recalling where he had made it.