Unknown Sayings of Jesus

Our subject is the lost sayings of our Lord, or--to give them their Greek name--the the agrapha, which means literally the unwritten sayings, saying which are not recorded in the four Gospels. As a matter of fact, we possess a not inconsiderable number of sayings which have come down to us outside the canonical Gospels, a fact of great importance not only for the study of the Gospels themselves, but also for the Church and its message . . . How does it come about, we immediately ask, that there are such things as sayings of Jesus outside the Gospels? Why were they left out of our Gospels? How did these two parallel streams of tradition arise, sayings in the Gospels and sayings outside them? --from chapter 1