There has never been one who knows yet does not act.
"Knowledge is the master of action; action is the effort of knowledge."
爱曰：「古人说知行做两个，亦是要人见个分晓。一行做知的功夫，一行做行的功夫，即功夫始有下落。」
先生曰：「此却失了古人宗旨也。某尝说知是行之主意，行是知之功夫。知是行之始，行是知之成。若会得时，只说一个知，已自有行在；只说一个行，已自有知在。古人所以既说一个知又说一个行者，只为世间有一种人，懵懵懂懂的任意去做，全不解思惟省察，也只是个冥行妄作，所以必说个知，方才才行得是。又有一种人，茫茫荡荡悬空去思索，全不肯着实躬行，也只是个揣摸影响，所以必说一个行，方才知得真。此是古人不得已补偏救弊的说话。若见得这个意时，即一言而足。」
Xu Ai said: "The ancients spoke of knowledge and action as two things, also to help people see clearly. One side works on the effort of knowledge, the other on the effort of action — only then does the effort have a place to land."
The Master said: "This loses the original intent of the ancients. I have said that knowledge is the master of action, and action is the effort of knowledge. Knowledge is the beginning of action; action is the completion of knowledge. If you understand this, then when you say knowledge, action is already included; when you say action, knowledge is already included. The reason the ancients spoke of both knowledge and action is simply that in the world there are some people who act blindly without thinking or reflecting — these are merely reckless actions, so one must speak of knowledge to make action correct. And there are others who think emptily without ever practicing — these are merely speculative guesses, so one must speak of action to make knowledge genuine. This was the ancients' remedy for correcting偏差 — if you understand this meaning, one sentence would suffice."
Two common types of people at work: those who charge ahead without thinking about direction (reckless action), and those who endlessly discuss without executing (empty speculation). The wisdom of unity of knowledge and action: thinking IS part of action, and action IS the verification of thinking.
"The reason the ancients spoke of both knowledge and action is simply that in the world there are some people who act blindly."
Yangming explains why the ancients separated knowledge and action — not because they are naturally two things, but to correct two types of偏差: those who act blindly without thinking (reckless action), and those who think emptily without practicing (speculation). Separating knowledge and action was a "remedy for correcting偏差," not a fundamental principle.
"If you understand this meaning, one sentence would suffice."
If one truly grasps the original meaning of the unity of knowledge and action, one sentence is enough — no need to separately discuss knowledge and action. This is Yangming's confidence in his own teaching: truth is simple.