“The katana has been the weapon of the samurai since time immemorial. Consider the inner meaning. Our blade is honed only on a single edge. Why? It is because with the dull edge against our flesh, the katana becomes our shield. This cannot happen with a two-edged sword, One day, in the midst of a melee, you may owe your life to the dull edge rather than the sharp one. Let this contrast remind you that attack and defense are one.”
Book:Cloud of Sparrows