来书云：「真知即所以为行，不行不足谓之知，此为学者吃紧立教，俾务躬行则可。若真谓行即是知，恐其专求本心，遂遗物理，必有暗而不达之处。抑岂圣门知、行并进之成法哉？」
知之真切笃实处即是行，行之明觉精察处即是知[1]，知、行工夫本不可离。只为后世学者分作两截用功，失却知、行本体，故有合一并进之说。「真知即所以为行，不行不足谓之知」，即如来书所云「知食乃食」等说可见，前已略言之矣。此虽吃紧救弊而发，然知、行之体本来如是，非以己意抑扬其间，姑为是说，以苟一时之效者也。
「专求本心，遂遗物理」，此盖失其本心者也。夫物理不外于吾心，外吾心而求物理，无物理矣；遗物理而求吾心，吾心又何物邪？心之体，性也，性即理也。故有孝亲之心，即有孝之理，无孝亲之心，即无孝之理矣；有忠君之心，即有忠之理，无忠君之心，即无忠之理矣。理岂外于吾心邪？晦庵谓：「人之所以为学者,心与理而已。心虽主乎一身，而实管乎天下之理；理虽散在万事，而实不外乎一人之心。」是其一分一合之间，而未免已启学者心、理为二之弊。此后世所以有「专求本心，遂遗物理」之患，正由不知心即理耳。夫外心以求物理，是以有暗而不达之处，此告子义外之说，孟子所以谓之不知义也。心一而已，以其全体恻怛而言，谓之仁；以其得宜而言，谓之义；以其条理而言，谓之理。不可外心以求仁，不可外心以求义，独可外心以求理乎？外心以求理，此知、行之所以二也。求理于吾心，此圣门知、行合一之教，吾子又何疑乎？
来信说：「真知就是用来行的，不行就不足以称为知，这对学者是切要的教法，使人务于躬行是可以的。若真的说行就是知，恐怕人们专求本心，便遗漏了事物之理，必有晦暗不通之处。这难道也是圣门知行并进的成法吗？」
知得真切笃实处就是行，行得明觉精察处就是知。知行的功夫本不可分离，只因后世学者分作两截用功，失掉了知行的本体，所以才有合一并进之说。「真知即所以为行，不行不足谓之知」，照来信「知食乃食」等说法即可看出，前面已大略说过了。这虽是为切要救弊而发，然而知行的本体本来就是这样，并非以己意抑扬其间、勉强立说以苟求一时之效。
「专求本心，遂遗物理」，这是失去了本心的人才有的说法。事物之理不外于我心，离开我心去求事物之理，就没有物理了；抛开物理去求我心，我心又是什么东西呢？心之体是性，性就是理。所以有孝亲之心，就有孝之理；没有孝亲之心，就没有孝之理。有忠君之心，就有忠之理；没有忠君之心，就没有忠之理。理难道在我心之外吗？晦庵说：「人之所以为学，心与理而已。心虽主于一身，而实管乎天下之理；理虽散在万事，而实不外乎一人之心。」在他这一分一合之间，未免已开了学者把心理分作二物的弊端。后世之所以有「专求本心，遂遗物理」的祸患，正由于不知道心就是理。离开心去求物理，所以有晦暗不通之处，这正是告子「义外」之说，孟子所以说他不知义。心只是一个：就其全体恻怛而言叫作仁，就其处置得宜而言叫作义，就其条理而言叫作理。不可离开心去求仁，不可离开心去求义，偏偏可以离开心去求理吗？离开心去求理，这就是知行分作二事的原因。在我心中求理，这才是圣门知行合一之教，您还有什么可疑的呢？
Your letter says: 'That true knowledge is for action, and without action knowledge does not count, is urgent teaching spurring learners to practice — well and good. But to say action itself is knowledge may drive men to seek the mind alone and neglect the principles of things, with dark places they cannot reach. Is that the established method of the school, knowledge and action advancing together?'
Knowledge at its most genuine and solid is action; action at its lucid and discerning is knowledge. The two works were never to be parted; only because later scholars split them in two and so lost the original substance of knowledge and action did the teaching of union and joint advance arise. That true knowledge is for action your own examples of eating show, as touched on before — a remedy urgently raised, yet the substance of knowledge and action was always thus, not my private tilt framed for passing effect.
'Seeking the mind alone, neglecting principles' is the speech of one who has lost his original mind. The principles of things are not outside my mind: outside the mind to seek them, there are none; dismissing them to seek the mind, what would the mind be? The mind's substance is nature, and nature is principle. With a filial heart there is the principle of filial piety; without the filial heart, no such principle. With a loyal heart, the principle of loyalty; without it, none. Is principle outside my mind? Zhu Xi said: learning is mind and principle; the mind, though master of one body, truly commands the world's principles; the principles, though scattered in the myriad affairs, truly do not lie outside one man's mind. In that one sundering and one joining he already opened the defect of making mind and principle two. Hence the later plague of 'seeking the mind alone, neglecting principles' — precisely from not knowing the mind is principle. Seeking principle outside the mind, dark impasses arise — Gaozi's 'rightness is external', which Mencius condemned as not knowing rightness. The mind is one: as wholehearted compassion it is called humaneness; as fitting measure, rightness; as ordered pattern, principle. One may not seek humaneness outside the mind, nor rightness — yet principle alone may be? Seeking principle outside the mind is why knowledge and action became two. Seeking principle in my mind is the school's teaching of their unity — what doubt remains for you?
- 此不见于《传习录上》，系晚年对知行合一说的深入阐发。