Much of the philosophy of Ni Ten Ryu is based around that of Miyamoto Musashi. In particular his collected letters which form the book "Go Rin No Sho" or "The Book of Five Rings".
Musashi was a Ronin, or samurai without allegiance to any lord, who was born in 1584. Musashi fought and defeated many men in the course of his life, he was never defeated himself. Musashi became skilled in much more than the sword, he was an accomplished painter and poet. In his later years he began to write his "guide for men who want to learn strategy", in the form of letters to his pupil.
"Ni Ten Ryu" means "Two Heavens School". Musashi sometimes marked his paintings "niten", meaning "Two Heavens", and it is thought that this may also be a reference to his preferred way of fighting, using two swords as the term for two swords is "nito".