The term ishin-denshin refers to the mutual understanding of feelings and thoughts directly from heart to heart, without the need for spoken words or written text. This concept originates from the Zen teaching of furyū-monji—the idea that spiritual truth is transmitted directly from heart to heart rather than relying on written scriptures or verbal explanations. It signified a state of enlightenment that transcends teachings conveyed through language. The character shin (or kokoro) corresponds to the English word "heart," while the character den signifies transmission or communication. It was originally written with a brush in the gyosho (semi-cursive) style.