Do/Michi [道]

The character/ideogram means "road; street; way; path; course; route; lane; the way (of proper conduct, etc.); one's way; morals; teachings (esp. Confucian or Buddhist); dogma; way; method; means." Also "Taoism."

This is the yin of martial arts. The yang is the physical combative aspects. Both together give you the one whole of all martial arts. You can see this division when you study the complete and fundamental principles of all martial systems, i.e. the principles of theory, physiokinetics, technique and philosophy. 

The goku-i also alludes symbolically and metaphorically to this division and within the overall division of yin-yang, each smaller part contributes its yin-yang to the whole for a complete practice of martial arts. This is the yin or way, path or method to the yang or jutsu, technique or application of the physical that is the combatives of fighting, combat and war. 

To leave any part out of the whole leaves the martial arts broken and lacking completeness or wholeheartedness. To be complete human kind must have both male and female - it is just nature. Nature involves two complementary but different halves to make things whole and this goes especially with martial arts. 

To have a practitioner of martial arts without the yin half leaves a brutal and often misguided application of techniques. It is leaving reality out for the sake of the fiction that is today's martial art community - mostly. 

To teach something that is not relevant to reality, i.e. self-defense or combatives, is to leave out the yang for the benefit of more yin. Humans abhor violence on the whole and will naturally avoid it. This tends toward acceptance of the more yin aspects leaving the yang to flounder on the shore out of the life sustaining water that is the whole of martial practices. 

Embrace the whole to achieve true martial arts. Leave the halves joined, moving and changing to be a martial art that is both art and practical for today's world still with violence as a part of the one whole that is mankind.