Cicero, De Inventione 2.33
“In every case, [the orator] should know the nature, the way of life, the interests or the fortune or any of those personal qualities which he might say were a cause for him to have committed the act he did or he should find fault with his character by reference to another kind of crime if there is no ability to bring up those of a similar crime.
If you are arguing that someone acted because of greed and you cannot show that the person you are accusing is greedy, you need to show that he has an affinity with other vices and, by implication, that it is not a surprise for someone who has acted either corruptly or greedily, or petulantly in other matters should have been wrong in this affair too. For every bit that detracts from a defendant’s honesty and authority diminishes the ease of his whole defense.”
Item in omni causa naturam aut victum aut studium aut fortunam aut aliquid eorum quae personis attributa sunt ad eam causam qua commotum peccasse dicet adiungere atque ex dispari quoque genere culparum, si ex pari sumendi facultas non erit, improbare animum adversari oportebit: si avaritia inductum arguas fecisse, et avarum eum quem accuses demonstrare non possis, aliis affinem vitiis esse doceas, et ex ea re non esse mirandum, qui in illa re turpis aut cupidus aut petulans fuerit, hac quoque in re eum deliquisse. Quantum enim de honestate et auctoritate eius qui arguitur detractum est, tantundem de facultate eius totius est defensionis deminutum.