Rabanus Maurus

Having forbidden swearing, He instructs us how we ought to speak, Let your speech be yea, yea; nay, nay. That is, to affirm any thing it is sufficient to say, ‘It is so:’ to deny, to say, ‘It is not so.’ Or, yea, yea; nay, nay, are therefore twice repeated, that what you affirm with the mouth you should prove in deed, and what you deny in word, you should not establish by your conduct.