"The major boxing arts are generally divided into three schools: Shaolin, Wudang, and Emei. As for the rest, there is a great variety, but more than half can be classified under these schools. Shaolin began with Damo’s two classics – Sinew Changing and Marrow Washing. Then Yue Fei in the Song Dynasty started what was called Xingyi Boxing, applying his own Bone Changing Classic and calling his art Xingyi [“form” & “intent”]. “Form” means the shape. “Intent” means the mental intention. What is expressed from the mind manifests in the hands and feet.
This boxing art has “five cores” and “twelve imitations”. The five cores are the five elements – metal, wood, water, fire, earth – which make the five boxing techniques of chopping, crashing, drilling, blasting, and crossing. The twelve imitations are the twelve animals: dragon, tiger, monkey, horse, alligator, rooster, hawk, kestrel, swallow, snake, eagle, and bear. Strive for the instincts and abilities of these twelve animals, and to be able to incorporate within yourself everything they can do. Thus it is said [in the Zhong Yong]: “Giving full expression to human nature, one is thus able to give full expression to the nature of animals.”
What should we understand to be the nature of this art? The chopping technique corresponds to metal, and in the body to the lungs. The crashing technique corresponds to wood, and in the body to the liver. The drilling technique corresponds to water, and in the body to the kidneys. The blasting technique corresponds to fire, and in the body to the heart. The crossing technique corresponds to earth, and in the body to the spleen. After practicing for a long time, it can dispel the ailments of the five organs. This is called “ingraining human nature”.
As for the animals, the dragon has the method of shrinking its body, the tiger has the fierceness of pouncing on prey, the monkey has the nimbleness of bounding up hillsides, the bear [alligator] has the quality of floating on water, and the other eight animals each have their own ingenuities. This is called “ingraining the nature of animals”.
When both human and animal natures are ingrained, then lifting and dropping, advancing and retreating, transforming without limit, will all as a result be performed with resourcefulness.
Obtaining a state of centered harmoniousness, the essence of the animal does not get discarded and yet the result is humanity.
Mind is united with the intention, the intention united with the energy, and the energy united with the power. These are the three internal unions. The shoulder is united with the hip, the elbow united with the knee, and the hand united with the foot. These are the three external unions. The internal and external unions merged together makes the six unions, and the result is courage.
Once these three things [resourcefulness, humanity, courage] are prepared, then in every action and movement, upper body and lower will be coordinated with each other, hand and foot will be aligned with each other, and you will be [from Mengzi, chapter 2a:] “nurturing your noble energy… until it is vast and strong”. It is within the Confucian concept of sincerity that can be found the key to these external shapes, the [from the Lun Yu, 15.3:] “single principle running through the whole thing”. These are the main ideas of Xingyi Boxing."
Translation by Paul Brennan of "A Detailed Look at the Theories of Xingyi, Bagua and Taichi" (詳論形意八卦太極之原理) by Sūn Lùtáng (孫祿堂), to be found in the Brennan Translation blog.