Below is an excerpt from Boxing Concepts Explained Authentically by Sun Lu Tang. The whole translation may be read here.
夫道者。陰陽之根。萬物之體也。其道未發。懸於太虛之內。其道已發。流行於萬物之中。 夫道一而已矣。在天曰命。在人曰性。在物曰理。在拳術曰內勁。所以內家拳術。有形意。八卦。太極。三派。形式不同。其極還虛之道則一也。易曰。一陰一陽之 謂道。若偏陰偏陽皆謂之病。夫人之一生。飲食之不調。氣血之不和。精神之不振。皆陰陽不和之故也。故古人。創內家拳術。使人潜心玩味。以思其理。身體力 行。以合其道。則能復其本來之性體。然吾國拳術。門派頗多。形式不一。運用亦異。畢生不能窮其數。歷世不能盡其法。余自幼年好習拳術。性與形意。八卦。太 極。三派之拳術相近。研究五十餘年。得其槪要。曾著形意八卦太極拳學已刊行世。今又以昔年所聞先輩之言。述之於書。俾學者得知其真意焉。三派拳術。形式不 同。其理則同。用法不一。其制人之中心。而取勝於人者則一也。按一派拳術之中。諸位先生之言論形式。亦有不同者。蓋其運用。或有異耳。三派拳術之道。始於 一理。中分為三派。末復合為一理。其一理者。三派亦各有所得也。形意拳之誠一也。八卦拳之萬法歸一也。太極拳之抱元守一也。古人云。天得一以清。地得一以 寧。人得一以靈。得其一而萬事畢也。三派之理。皆是以虛無而始。以虛無而終。所以三派諸位先生所練拳術之道。能與釋儒道三家。誠中。虛中。空中之妙理。合 而為一者也。余深恐諸位先生之苦心精詣。久而淹没。故述之以公同好。惟自愧學術譾陋。無文。或未能發揮諸位先生之妙旨。望諸同志。隨時增補之。以發明其道 可也。
The Way is the root source of the passive and active aspects, and is the essence of all things. When the Way was not yet expressed, it was suspended in the Void, and since the Way was expressed, it has been flowing within all things. [from Mengzi, chapter 3a:] “The Way is One, and that is all.” In Nature it is called fate. In people is called nature. In things it is called principle. In boxing arts it is called internal power, hence the “internal” schools of boxing arts, in which there are the three systems of Xingyi, Bagua, and Taiji.
Their postures are different, but at their limit is the Way of returning to emptiness, and in this way they are the same. The Book of Changes says: “The Way is the passive AND the active.” To incline toward the passive OR the active is wrong. Throughout your life, if your eating and drinking are not regulated, your energy and blood not harmonized, or your essence and spirit not roused, all these will put your passive and active aspects out of balance as a result. Therefore people long ago created the internal schools of boxing arts to get you to concentrate and ponder upon the theory and to get your body’s movements to conform with the Way, thus enabling you to return to your original essence.
In our nation’s boxing arts, there are so many schools, and their postures and applications are so varied. In a whole lifetime, you would not be able to even count them all, and even if given all the time in the world, we would not be able to exhaustively examine their methodologies. When I was young, I was fond of practicing boxing arts, and my disposition is toward the three systems of Xingyi, Bagua, and Taiji. I have studied for more than fifty years and have obtained their essentials. I have written books for each, which have already been published. Now I am also taking what I have been told over the years by previous generations of teachers and putting it all into a book so that students may obtain their authentic ideas.
The postures of the three boxing arts are different, but their theory is the same. Their applications may be different, yet they have controlling the opponent as their core, and so they defeat opponents in the same way. Within a single system of boxing, everyone’s explanations and postures are not identical, owing either to how they use them or just to differences from one person to another.
The methods of these three boxing systems begin with the principle of oneness, divide from there into the three distinct branches, then end up united again in the principle of oneness. Each of the three systems has its own way of oneness: Xingyi Boxing’s sincerity is a pure oneness, Bagua Boxing’s infinity of techniques is a returning to oneness, and Taiji Boxing’s embracing of original nature is a maintaining of oneness. It was said by men long ago [Laozi – Daodejing, chapter 39 / Zhuangzi, chapter 12]: “The sky obtains oneness by being clear. The ground obtains oneness by being firm. Man obtains oneness by being smart.” / “Obtaining the One, all things are accomplished.” The principle of the three systems is always to begin in emptiness and to end in emptiness. Therefore the way the teachers of the three boxing arts practiced can be equivalent to the three schools of Confucianism, Daoism, and Buddhism – with their special theories of sincerity, undifferentiation, and non-identity – merging to become one.
I deeply fear that all the effort those teachers put into perfecting these things will be wasted with the passing of time. Thus I transmit it to share with those who will appreciate it. However, I am ashamed of my own learning, how shallow, ignorant, and illiterate I am, and that I cannot give full expression to their wonderful ideas. It would be good if their methods can be further elaborated upon, and so I hope my comrades will make more information available as it comes to light.
– written by Sun Fuquan of Wan county, Hebei, 1923