The Magician's Oath

The Magician's Oath (though it may vary, 'The Oath' takes the following, or similar form):

"As a magician I promise never to reveal the secret of any illusion to a non-magician, unless that one swears to uphold the Magician's Oath in turn. I promise never to perform any illusion for any non-magician without first practicing the effect until I can perform it well enough to maintain the illusion of magic."