Actually in order to glorify history he says that the difference between it and declamatory writing is as great as that between real buildings or furniture and the betweens and compositions we see in scene-paintings.
In the second place he says that the mere collection of the material required for a history is a more serious task than complete course of study of the art of declamatory speaking.
He himself, he tells us, had incurred such expense and been put to so much trouble in collecting his notes from <a class="linkToPlace" target="_blank" href="/place?placename=Assyria&groupId=375&placeId=707">Assyria</a> [?] and inquiring into the manners and customs of the Ligurians, Celts, and Iberians that he could not hope that either his own testimony or that of others to this would be believed.
One would like to ask this writer whether he thinks that to sit in town collecting notes and inquiring into the manners and customs of the Ligurians and Celts involves more trouble and expense than an attempt to see the majority of places and peoples with one's own eyes.
Which again is most troublesome, to inquire from those present at the engagements the details of battles by land and sea and of sieges, or to be present at the actual scene and experience oneself the dangers and vicissitudes of battle?
In my opinion the difference between real buildings and scene-paintings or between history and declamatory speech-making is not so great as is, in the case of all works, the difference between an account founded on participation, active or passive, in the occurrences one composed from report and the narratives of others.
But Timaeus, having no experience of the former proceeding, naturally thinks that what is really of smallest importance and easiest is most important and difficult, I mean the collection of documents and inquiry from those personally acquainted with the facts.
And even in this task men of no experience are sure to be frequently deceived. For how is it possible to examine a person properly about a battle, a siege, or a sea-fight, or to understand the details of his narrative, if one has no clear ideas about these matters?
For the inquirer contributes to the narrative as much as his informant, since the suggestions of the person who follows the narrative guide the memory of the narrator to each incident,
and these are matters in which a man of no experience is neither competent to question those who were present at an action, nor when present himself to understand what is going on, but even if present he is in a sense not present.
Walbank Commentary