Poetry and Truth: Forgeries

I can imagine a forger clever enough to imitate another's signature so exactly that a handwriting expert would swear in court that it was genuine, but I cannot imagine a forger so clever that he could imitate his own signature inexactly enough to make a handwriting expert swear that it was a forgery. (Or is it only that I cannot imagine the circumstances in which anyone could want to do such a thing?)

(W.H. Auden, Dichtung and Wahrheit X)

It is almost like taking Polonius' advice to Laertes in Shakespeare's Hamlet as inevitable: not only "to thine own self be true" but one cannot but be true to oneself. Although one's signature is duplicable, one can only falsify others and not oneself. It is an interesting idea to express in handwriting. Although I am not sure it is true, and, in fact, Auden himself expresses parenthetical doubts that not being able to think of one who could falsify oneself is due to a lack of imagination.