Cicero is accused on the strength of his own private letters.

It is because we have not the private letters of other persons that they are not so accused.

The courtesies of the world exact, I will not say demand, certain deviations from straightforward expression; and these are made most often in private conversations and in private correspondence.

Cicero complies with the ways of the world; but his epistles are no longer private, and he is therefore subjected to charges of falsehood.

It is because Cicero's letters, written altogether for privacy, have been found worthy to be made public that such accusations have been made.