Aishū Ikōsai

Aishū Isōsai (1452–1538) is regarded as one of the most important figures in the history of Japanese swordsmanship and is venerated as a sword saint who founded the In-ryū (Shadow School) swordsmanship, thereby establishing the source of Japanese sword arts. He is said to have been born in the southern part of present-day Mie Prefecture (the former province of Minami-Ise), and after traveling throughout the nation to study the sword arts of various regions and mastering his own unique sword principles, he established the In-ryū.

From his youth, Isōsai undertook journeys to practice martial strategy, and is said to have even traveled to Ming China. A legend persists that he received divine revelation at the seashore of Udō Shrine in Miyazaki Prefecture, Kyushu, and thereby grasped the true essence of the sword; this experience is said to have become the source of the sword concept of "In" (陰, the yin aspect of yin-yang). "In" signifies the principle of the sword that, like water flowing in accordance with the earth, changes formlessly in response to the opponent's movements.

The most renowned successor to the In-ryū was Kaminoizumi Nobutsuna (Ise no Kami), who further developed the In-ryū and founded the Shinkage-ryū (New Shadow School). Furthermore, the Yagyū Shinkage-ryū branched from the Shinkage-ryū and became the instructional school of the Tokugawa shogunate. Additionally, the renowned swordmaster Miyamoto Musashi's Niten Ichiryu is said to have been influenced by the In-ryū, and Aishū Isōsai's sword philosophy indirectly influenced numerous schools of Japanese swordsmanship.

Although Isōsai's life abounds in legendary elements and his true nature remains unclear in many respects, the propagation routes centered in Kyushu and Shikoku attest to the reality of his existence. Isōsai, said to have attained the exceptional longevity of 86 years—extraordinary for his time—pursued a sword philosophy of "sword principles that transcend form," which carries universal value inherited by contemporary martial arts and martial disciplines.