The philosophy of Ken Zen Ichi, which translates as “The Sword and Zen are One,” focuses on Zen Master Takuan Soho's letter to fencing master Yagyu Munenori in the 16th century. The excerpt clarifies that true mastery of the art of swordsmanship, as defined in Takuan's Taikai, requires the complete unification of spirit (Ri) and technique (Waza). Takuan argues that many practitioners fail to achieve mastery because they prioritize only the physical, remaining prisoners of their own ego and its prejudices, which cloud their understanding of both the martial art and reality. According to this view, true victory lies in overcoming one's ego through the emptying of the spirit (Ku), essential to facing any human challenge. Takuan's explanations of spiritual education played a pivotal role in the transformation of Bujutsu (deadly martial art) into Budo (Way of Life).