"However hard you try, Jesus said, you cannot avoid some reference to God, for the whole world is God's world and you cannot eliminate Him from any of it. If you vow by 'heaven', it is God's throne; if by 'earth' it is his footstool; if by 'Jerusalem' it is his city, THE CITY OF THE GREAT KING. If you swear by your head, it is indeed yours in the sense that it is nobody else's, and yet it is God's creation and under God's control. You cannot even change the natural colour of a single hair, black in youth and white in old age."
--John R. W. Stott, from The Message of the Sermon on the Mount (Inter-Varsity Press, 1978, p. 101)