terms of the theory of historiography:

We judge that what happened at the siege of Antioch cannot possibly
be told by anyone, because, among those who were there, no one can be
found who could have observed everything that took place throughout
the city, or who could understand the entire event in a way that
would enable him to represent the sequence of actions as they took
place.

At the beginning of the fourth book of the Gest Dei, Guibert's
defense of his absence is again intertextual, but openly polemic as
well, as he declares the battle between modern Christian writing
(saints lives and John III.32) and ancient pagan authority (Horace,
Ars Poetica 180-181) no contest:

If anyone objects that I did not see, he cannot object on the grounds
that I did not hear, because I believe that, in a way, hearing is
almost as good as seeing. For although:

Less vividly is the mind stirred by what finds entrance through the
ears than by what is brought before the trusty eyes.[29]

Yet who is unaware that historians and those who wrote the lives of
the saints wrote down not only what they had seen, but also those
things they had drawn from what others had told them? If the
truthful man, as it is written, reports "what he has seen and heard,"
then his tale may be accepted as true when he describes what he has
not seen, but has been told by reliable speakers.

Guibert then goes on to challenge those who object to do the job