The metaphor functions on several levels: 1) in any argument, there
is your side, and the other side, but if you're ever going to get past
that and find agreement, or understanding, you must be open to a third
idea, a compromise or intercession. 2) a real three-bladed or three-edged
sword creates a wound that does not close again; knowledge, once received,
and understood, is permanent and changes you forever.

jms