Institutio Oratoria

Every artificial proof consists either of indications, arguments or examples. I am well aware that many consider indications to form part of the arguments. My reasons for distinguishing them are twofold. In the first place indications as a rule come under the head of inartificial proofs: for a bloodstained garment, a shriek, a dark blotch and the like are all evidence analogous to documentary or oral evidence and rumours; they are not discovered by the orator, but are given him with the case itself.

My second reason was that indications, if indubitable, are not arguments, since they leave no room for question, while arguments are only possible in controversial matters. If on the other hand they are doubtful, they are not arguments, but require arguments to support them.