5:33-37 There is no reason to consider that solemn oaths in a court of justice, or on other proper occasions, are wrong, provided they are taken with due reverence. But all oaths taken without necessity, or in common conversation, must be sinful, as well as all those expressions which are appeals to God, though persons think thereby to evade the guilt of swearing. The worse men are, the less they are bound by oaths; the better they are, the less there is need for them. Our Lord does not enjoin the precise terms wherein we are to affirm or deny, but such a constant regard to truth as would render oaths unnecessary.
36. Neither shalt thou swear by thy head, because thou canst not make one hair white or black—In the other oaths specified, God's name was profaned quite as really as if His name had been uttered, because it was instantly suggested by the mention of His "throne," His "footstool," His "city." But in swearing by our own head and the like, the objection lies in their being "beyond our control," and therefore profanely assumed to have a stability which they have not.