[A maxim of the fighting/martial arts. In one form or another we all have heard this told to represent the hard and/or soft of practice and training. The goku-i refers to hard and soft in the third tome, "The manner of drinking and spitting is either soft or hard." This direct reference is also indicative of Yin-n-Yang or in Japanese, "in-n-yo."
Indirectly the other tomes of the goku-i such as balance, shifting, strike/not-strike, see and hear, etc. all come from the basic tenant of Yin-n-Yang but only in less extremes of soft and hard. The many levels, i.e. extremes poles to all the variances and degrees of intensity to the actual balance or that equilibrium we strive to achieve in our Way of the Empty Hand.]