Timaeus says, that as a rule which is defective in length and breadth but possesses the essential quality of a rule must still be called a rule, but when it has no approach to straightness or any quality akin to straightness, must be called anything rather than a rule,
so in the case of historical works, when they are defective in style, treatment, or any other particular quality but still strive to ascertain the truth they may claim to be styled histories, but when they fall away from truth have no longer any claim to this name.
I quite agree with him that truth is the leading quality in such books, and I made the same statement, writing as follows, that in the case of a living body if the eyes are put out the whole becomes useless, so if you take away truth from history what remains is but an unprofitable fable.
however, that there are two kinds of falsehood, one the consequence of ignorance and the other deliberate,
and that we should accord pardon to those who fall away from the truth owing to ignorance, but should refuse to forgive deliberate lying. This point being settled I affirm
that the difference is very wide between such falsehood as is the result of ignorance and such as is deliberate, the one admitting of pardon and kindly correction but the other deserving implacable condemnation.
And one finds that Timaeus himself is a chief sinner in this respect, as I will now prove.
Walbank Commentary