Institutio Oratoria

But the consideration as to what argument should be put forward requires no less care than the consideration of the manner in which we are to prove that which we have put forward. Indeed in this connexion invention, if not the most important, is certainly the first consideration. For, just as weapons are superfluous for one who does not know what his target is, so too arguments are useless, unless you see in advance to what they are to be applied. This is a task for which no formal rules can be laid down.

When Alexander destroyed Thebes, he found documents showing that the Thebans had lent a hundred talents to the Thessalians. These documents he presented to the Thessalians as a reward for the assistance they had given him in the campaign. Subsequently the Thebans, after the restoration of their city by Cassander, demanded that the Thessalians should repay the money.