Following his logic of Mutô Dori being Butô Dori, Sensei in a recent class said that “Mushin is Bushin (Bujin)”.
As you know, the word “Bujinkan” is sometimes written as “Bushinkan” (in the old translations form Japan).
This opens up a new series of possible interpretations.
As we know 無 is pronounced Mu or Bu and this allows sensei to play with the words as the sounds are similar.
So if Bu 無 replaces Bu 武 in Bushinkan, then the “house of the war god” becomes the “place where we learn to have no intention”. Maybe this is what sensei want us to understand when he speaks about “natural movement”?
But as I was not attending this class my guess is as good as yours.
In fact, there are many possible other interpretations with those sounds.
Hereafter I wrote the ones that are the most logical for me:
- Bushin the normal one: 武神 = god of war
- Bushin the human one: 武人 = military man:
- Bushin the new one: 武心 = warrior heart/spirit
- Bushin another one: 無神 = god of nothingness
- Bushin another one: 無人 = non human
- Bushin another one: 無心 = no spirit
Funnily “六”, “6” can also be pronounced Mu, and there are 6 logical interpretations above! But it is only a chance as Mu has (at least) 13 different meanings, Bu 19 , Jin 18 and shin 36!
If we take the general accepted meaning for Mushin 無心 of “no spirit” we have to understand also that in spirituality it also refers to “innocence” or “(to be) free from obstructive thoughts”. When you reach the state of Mushin, you are like this 3 year old infant acting without preconceived idea. Which is the true Sanshin.
So let’s consider “Mushin” as being “6+spirits”, as “6” is shiki 識 (consciousness) which is the sixth element, then the objective of the bujinkan is to reach the level of consciousness where natural movement can be expressed without thinking.
All these understandings being possible, this is why we can say that Mushin is Sanshin!
(sorry for the headache)
Shiro Kuma 
