爱因未会先生「知行合一」之训，与宗贤、惟贤往复辩论，未能决，以问于先生。
先生曰：「试举看。」
爱曰：「如今人尽有知得父当孝、兄当弟者，却不能孝、不能弟，便是知与行分明是两件。」
先生曰：「此已被私欲隔断，不是知行的本体了。未有知而不行者；知而不行，只是未知。圣贤教人知行，正是安复那本体，不是著你只恁的便罢。故《大学》指个真知行与人看，说『如好好色，如恶恶臭』。见好色，属知；好好色，属行。只见那好色时，已自好了；不是见了后，又立个心去好。闻恶臭，属知；恶恶臭，属行。只闻那恶臭时，已自恶了；不是闻了后，别立个心去恶。如鼻塞人虽见恶臭在前，鼻中不曾闻得，便亦不甚恶，亦只是不曾知臭。就如称某人知孝、某人知弟，必是其人已曾行孝、行弟，方可称他知孝、知弟。不成只是晓得说些孝、弟的话，便可称为知孝、弟？又如知痛，必已自痛了，方知痛；知寒，必已自寒了；知饥，必已自饥了。知行如何分得开？此便是知行的本体，不曾有私意隔断的。圣人教人必要是如此，方可谓之知；不然，只是不曾知。此却是何等紧切著实的工夫？如今苦苦定要说知行做两个，是甚么意？某要说做一个，是什么意？若不知立言宗旨，只管说一个、两个，亦有甚用？」
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：知是行的主意，行是知的功夫；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，懵懵懂懂地任意去做，全不解思惟省察，也只是个冥行妄作，所以必说个知，方才行得是；又有一种人，茫茫荡荡悬空去思索，全不肯著实躬行，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。今人却就将知行分作两件去做，以为必先知了，然后能行。我如今且去讲习、讨论做知的工夫，待知得真了，方去做行的工夫；故遂终身不行，亦遂终身不知。此不是小病痛，其来已非一日矣。某今说个知行合一，正是对病的药。又不是某凿空杜撰，知行本体原是如此。今若知得宗旨时，即说两个亦不妨，亦只是一个。若不会宗旨，便说一个，亦济得甚事？只是闲说话。」
徐爱因为未能领会先生「知行合一」的训示，与黄宗贤、顾惟贤反复辩论，不能决定，便来请教先生。
先生说：「试着举个例子看。」
徐爱说：「如今尽有人晓得对父当孝、对兄当悌，却不能孝、不能悌，可见知与行分明是两件事。」
先生说：「这是已被私欲隔断了，不是知行的本体了。没有知道了却不去做的事；知而不行，只是还不曾真知。圣贤教人知行，正是要安复那个本体，不是让你只这样说说便罢。所以《大学》举出真知真行给人看，说『如好好色，如恶恶臭』。看见美色，属知；喜好美色，属行。只在那看见美色的时候，已经自然喜好上了；不是见了之后，又另立一个心去喜好。闻到恶臭，属知；厌恶恶臭，属行。只在那闻到恶臭的时候，已经自然厌恶了；不是闻了之后，另立一个心去厌恶。如鼻塞的人虽然见恶臭在前，鼻中没有闻到，便也不很厌恶，也只是不曾知臭。就像说某人知孝、某人知悌，必是那人已经行过孝、行过悌，才可称他知孝、知悌。不成只是晓得说些孝悌的话，便可称为知孝知悌？又比如知痛，必是自己痛过了，才知痛；知寒，必是自己冷过了；知饥，必是自己饿过了。知与行怎么分得开？这便是知行的本体，不曾被私意隔断的。圣人教人必须如此，才可称为知；不然，只是不曾知。这是何等紧切著实的功夫？如今苦苦定要把知行说成两个，是什么意思？我要说成一个，又是什么意思？若不知道立言的宗旨，只管争一个、两个，又有什么用？」
徐爱说：「古人把知行说成两个，也是要人分辨明白。一边做知的功夫，一边做行的功夫，功夫才有下手处。」
先生说：「这却失了古人的宗旨。我曾讲：知是行的主意，行是知的功夫；知是行之始，行是知之成。若领会得，只说一个知，已自有行在；只说一个行，已自有知在。古人之所以既说一个知、又说一个行，只因为世间有一种人，懵懵懂懂任意去做，全不懂得思维省察，只是冥行妄作，所以必须说个知，才行得对；又有一种人，茫茫荡荡悬空去思索，全不肯切实躬行，只是捕风捉影，所以必须说一个行，才知得真。这是古人不得已补偏救弊的说法，若明白了这个意思，一句话就够了。今人却把知行分成两件事去做，以为必须先知了，然后能行。我如今且去讲习讨论做知的功夫，等知得真了，才去做行的功夫；于是终身不行，也就终身不知。这不是小毛病，由来已不是一天了。我如今说个知行合一，正是对症的药。并不是我凭空杜撰，知行的本体原是如此。如今若懂得宗旨，说两个也不妨，其实只是一个。若不懂宗旨，便说一个，又顶什么用？只是闲话。」
Xu Ai, unable to grasp the Master's teaching of "the unity of knowledge and action", had debated back and forth with Huang Zongxian and Gu Weixian without resolution, and brought the question to the Master.
The Master said: "Give an example."
Xu Ai said: "Nowadays there are plenty who know one should be filial to a father and respectful to an elder brother, yet cannot be filial or respectful. Clearly knowledge and action are two things."
The Master said: "That is knowledge and action severed by selfish desire — no longer their original substance. There is no knowing without acting; knowing without acting is simply not yet knowing. When sages teach knowledge and action, it is precisely to restore that original substance, not to let you stop at talk. Hence the Great Learning shows what genuine knowing and acting are: 'as one loves a beautiful color, as one hates a bad odor.' Seeing a beautiful color is knowledge; loving it is action. The moment one sees it, one has already loved it — one does not first see it and then set up a separate mind to love it. Smelling a bad odor is knowledge; hating it is action. The moment one smells it, one already hates it. A man with a blocked nose may see the foul thing before him yet smell nothing, and so does not much hate it — he simply has not known the stench. To call someone knowing in filial piety or brotherly respect, he must already have practiced them — could mere ability to talk about them qualify? Again, to know pain one must have felt pain; to know cold, one must have been cold; to know hunger, one must have been hungry. How can knowledge and action be separated? This is the original substance of knowledge and action, unsevered by private opinion. The sages taught that only this counts as knowing; otherwise one simply has not known. What urgent and solid work this is! To insist stubbornly on making them two — what does that mean? And my insisting on one — what does that mean? Without grasping the point of the teaching, arguing one or two is useless."
Xu Ai said: "The ancients spoke of them as two so people would see clearly, working at knowledge on one side and action on the other, so the effort would have a place to land."
The Master said: "That misses the ancients' intent. I have said: knowledge is the design of action, action the work of knowledge; knowledge is the beginning of action, action the completion of knowledge. Grasp this, and to speak only of knowledge already implies action; to speak only of action already implies knowledge. The ancients spoke of both only because some people act muddle-headedly at random, without reflection or self-examination — mere blind doing — so knowledge had to be stressed for action to be right; and others speculate wildly in mid-air, never deigning to practice — mere shadow-chasing — so action had to be stressed for knowledge to be true. Those were remedies for partial ills; once the intent is seen, one word suffices. People today split them into two tasks, thinking one must first know, then act: 'Let me first study and discuss until my knowledge is true, then act' — and so they never act to the end of their lives, and never know either. This is no small sickness, nor of one day's standing. My teaching of the unity of knowledge and action is the medicine for exactly this illness. I did not invent it: the original substance of knowledge and action is thus. Once the point is grasped, even speaking of two does no harm — they are still one. Without it, even speaking of one is idle talk."