Charles Ellicott Commentary Matthew 5:36

"Neither shalt thou swear by thy head, for thou canst not make one hair white or black." — Matthew 5:36 (ASV)

By your head—This is apparently chosen as an extreme instance of a common oath in which people found no reference to God. Yet here, too, nothing but an implied reference to Him makes it a fitting oath at all. He made us, and not we ourselves, and the hairs of our head are not only numbered but are also subject in all their changes to His laws, and not to our own will.