Institutio Oratoria

But if the prosecution do allege some motive, he will either say that the motive alleged is

false or inadequate or unknown to the accused. For it is possible that a man may be quite ignorant of motives imputed to him. He may not, for example, have known whether the man whom he is accused of having killed had appointed him his heir or intended to prosecute him. All else failing, we may urge that motives are not necessarily of importance. For what man is there who is not liable to the emotions of fear, hatred or hope, and yet numbers of persons act on these motives without committing crime?