Persons deaf and dumb from birth were, in contemplation of the common law, presumed to be idiots. The experience of modern educators has taught that deaf mutes are possessed of more mental capacity and are capable of higher mental culture than was formerly-supposed. However, this ancient presumption of the common law still clings so far as to cast upon the party offering the witness the duty of showing that he is possessed of sufficient understanding. But if he be shown to possess sufficient mental capacity to understand and appreciate the nature and obligation of an oath, and to communicate, by means of writing, or by signs with the aid of an interpreter, in an intelligent manner, the facts concerning which he is called to testify, he is a competent witness. Some authorities assert that the written method is the better; others hold that method most satisfactory by which he can make himself best understood; while other decisions declare either method allowable.

23 Scruggs vs. State, 90 Tenn., 81.