In his thirty-fourth book Timaeus says, "Living away from home at <a class="linkToPlace" target="_blank" href="/place?placename=Athens&groupId=379&placeId=715">Athens</a> for fifty years continuously, and having, as I confess, no experience of active service in war or any personal acquaintance with places."
So that, when he meets with such matters in his history, he is guilty of many errors and misstatements, and if he ever comes near the truth he resembles those painters who make their sketches from stuffed bags.
For in their case the outlines are sometimes preserved but we miss that vividness and animation of the real figures which the graphic art is especially capable of rendering. The same is the case with Timaeus and in general with all who approach the work in this bookish mood.
We miss in them the vividness of facts, as this impression can only be produced by the personal experience of the author. Those, thing, who have not been through the events themselves do not succeed in arousing the interest of their readers.
Hence our predecessors considered that historical memoirs should possess such vividness as to make one exclaim when author deals with political affairs that he necessarily had taken part in politics and had experience of what is wont to happen in the political world, when he deals with war that he had been in the field and risked his life, and when he deals with private life that he had reared children and lived with a wife, and so regarding the other parts of life.This quality can naturally only be found in those who have been through affairs themselves and have acquired this sort of historical knowledge. It is difficult, perhaps, to have taken a personal part and been one of the performers in every kind of event, but it is necessary to have had experience of the most important and those of commonest occurrence.
Walbank Commentary