Despite the close association between Zen and swordsmanship in the popular mind, the Mikkyo sect of Buddhism was actually much more closely connected with the classical Japanese martial arts. Most of the Japanese sword traditions include instruction in Mikkyo at their higher levels, or once did before that information was lost.
“Mushin” is a Zen-influenced concept in the Japanese martial arts, particularly the art of swordsmanship. The opponent moves to attack you and you suddenly perform a technically perfect counter that cuts him down with a single strike, yet you never consciously think about what strategy you're going to use. That's called “mushin” or “non-thought.”
In my last blog, I discussed the concept of “ken zen ichi nyo,” or “the sword and Zen are one.” While this sentence is not entirely accurate in the historical sense, it does have some validity as an analysis of the type of thinking required to survive a sword combat.
“The Mysterious Record of Immovable Wisdom” is a work by the very influential Zen master Takuan Soho, in which he uses the mentality of sword combat as an analogy for the Zen mindset. He chose this particular analogy because the work was written for a student of his who was a famous sword instructor. In all likelihood, if he had been writing to a musician he would have used music as an analogy just as readily.
This is essentially a cliché of Japanese swordsmanship, often repeated by people without any extensive experience of either Zen or swordsmanship. Historically, most Japanese swordsmen were not particularly familiar with Zen Buddhism, but would be expected to have some understanding of Mikkyo or “esoteric” Buddhism. Mikkyo contains a number of magic charms and spells to overcome fear or survive dangerous situations, so warriors were interested in using Mikkyo for what they believed to be practical purposes. Zen enlightenment was, to put it mildly, above their pay grade.