Excerpt from <span class="chinese-post-title">拳意述真自序</span> the Author's Preface in <span class="chinese-post-title">拳意述真</span> Boxing Concepts Explained Authentically

一則

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。

孫祿堂

"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. It says in the Book of Changes [Great Treatise, part 1]: “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."

Excerpt from the translation by Paul Brennan, to be found in the Brennan Translation blog.