The peculiar function of history is to discover, in the first place, the words actually spoken, whatever they were, and next to ascertain the reason why what was done or spoken led to failure or success.
For the mere statement of a fact may interest us but is of no benefit to us: but when we add the cause of it, study of history becomes fruitful.
For it is the mental transference of similar circumstances to our own times that gives us the means of forming presentiments of what is about to happen, and enables us at certain times to take precautions and at others by reproducing former conditions to face with more confidence the difficulties that menace us.
But a writer who passes over in silence the speeches made and the causes of events and in their place introduces false rhetorical exercises and discursive speeches, destroys the peculiar virtue of history . And of this Timaeus especially is guilty, and we all know that his work is full of blemishes of the kind.
Walbank Commentary