All right, Luther cracks me up. In his short introduction, addressed directly to Erasmus, Luther comments on why he took so long to write De Servo Arbitrio (on the enslavement of the will). The first reason is that Erasmus' book was so tame that it didn't excite Luther enough ("you discuss the matter throughout with quite remarkable restraint, by which you have prevented my wrath waxing hot against you"). The second was that Erasmus didn't write anything new - every argument he had for free-will had already been said and (mostly) answered by Luther.
Luther comedically laments by comparing his writing talents to Erasmus', until he remembers that he has the Scriptures at his disposal. With a wonderful turn, Luther proclaims he is glad that Erasmus turned his pen to this subject - because if the greatest and most eloquent writer alive could not convince him to turn aside from Scripture's plain teaching, then nothing could.
Finally, Luther asks, "But may I ask you, my dear Erasmus, to bear with my want of eloquence, as I in these matters bear with your want of knowledge." Ha. Oh for the days.