Dr. Masaaki Hatsumi, Soke, (Grandmaster) of Bujinkan Budo Taijutsu
Real ninjutsu is not for assassins or wrongdoers, but for those who wish to cultivate perseverance and endurance in order to find better ways of leading a happy life. Looking back over nine hundred years of history, ninjutsu was understood as shinobu ho or a way of perseverance and endurance; and then came to be called ninjutsu (nin meaning perseverance and jutsu meaning an art. ) The well-tried ninjutsu developed into ninpo, which embraces a religious and philosophical concept as well as the art of war.
(-Hatsumi Soke, from Essence of Ninjutsu, ch.6)