But if the written laws contradict one another, then the connexion of art is such, and most of its principles are so connected and linked together, that the rules which we a little while ago laid down for cases of ambiguity, and which have just been given with reference to the letter and spirit of the law, may be all transferred to this third division also. For the topics by which, in the case of an ambiguous expression, we defended that meaning which is favourable to our argument must also be used to defend the law which is favourable to us when there are inconsistent laws. In the next place, we must contrive to defend the spirit of one law, and the letter of the other. And so the rules which were just now given relating to the spirit and letter of the law may all be transferred to this subject.
XL. I have now explained to you all the divisions of oratory which have prevailed, as laid down by the academy to which we are devoted, and if it had not been for that academy they could not have been discovered, or understood, or discussed. For the mere act of division, and of definition, and the distribution of the partitions of a doubtful question, and the understanding the topics of arguments, and the arranging the argumentation itself properly, and the discerning what ought to be assumed in arguing, and what follows from what has been assumed, and the distinguishing what is true from what is false, and what is probable from what is incredible, and refuting assumptions which are not legitimate, or which are inappropriate, and discussing all these different points either concisely as those do who are called dialecticians, or copiously as an orator should do, are all fruits of the practice in disputing with acuteness and speaking with fluency, which is instilled into the disciples of that academy. And without a knowledge of these most important arts how can an orator have either energy or variety in his discourse, so as to speak properly of things good or bad, just or unjust, useful or useless, honourable or base?
Let these rules then, my Cicero, which I have now explained to you, be to you a sort of guide to those fountains of eloquence, and if under my instruction or that of others you arrive at them, you will then acquire a clearer understanding of these things and of others which are much more important.
C.F. I will strive to arrive at them with great eagerness, my father; and I do not think that there is any greater advantage which I can derive even from your many excellent kindnesses to me.
This little piece was composed by Cicero as a sort of preface to his translation of the Orations of Demosthenes and Aeschines de Corona; the translations themselves have not come down to us.