来书云：「道之大端易于明白，所谓『良知、良能，愚夫愚妇可与及者』。至于节目时变之详，毫厘千里之谬，必待学而后知。今语孝于温凊定省，孰不知之？至于舜之不告而娶，武之不葬而兴师，养志、养口，小杖、大杖，割股、庐墓等事，处常、处变，过与不及之间，必须讨论是非，以为制事之本，然后心体无蔽，临事无失。」
「道之大端易于明白」，此语诚然。顾后之学者忽其易于明白者而弗由，而求其难于明白者以为学，此其所以「道在迩而求诸远，事在易而求诸难」也。孟子云：「夫道若大路然，岂难知哉？人病不由耳。」良知、良能，愚夫、愚妇与圣人同；但惟圣人能致其良知，而愚夫、愚妇不能致，此圣、愚之所由分也。节目时变，圣人夫岂不知？但不专以此为学。而其所谓学者，正惟致其良知，以精审此心之天理，而与后世之学不同耳。吾子未暇真知之致，而汲汲焉顾是之忧，此正求其难于明白者以为学之蔽也。
夫良知之于节目时变，犹规矩、尺度之于方圆、长短也；节目时变之不可预定，犹方圆、长短之不可胜穷也。故规矩诚立，则不可欺以方圆，而天下之方圆不可胜用矣；尺度诚陈，则不可欺以长短，而天下之长短不可胜用矣；良知诚致，则不可欺以节目时变，而天下之节目时变不可胜应矣。毫厘千里之谬，不于吾心良知一念之微而察之，亦将何所用其学乎？是不以规矩而欲定天下之方圆，不以尺度而欲尽天下之长短，吾见其乖张谬戾，日劳而无成也已。
吾子谓：「语孝于温凊定省，孰不知之？」然而能致其知者鲜矣。若谓粗知温凊定省之仪节，而遂谓之能致其知，则凡知君之当仁者，皆可谓之能致其仁之知，知臣之当忠者，皆可谓之能致其忠之知，则天下孰非致知者邪？以是而言，可以知致知之必在于行，而不行之不可以为致知也明矣。知、行合一之体，不益较然矣乎？
夫舜之不告而娶，岂舜之前已有不告而娶者为之准则，故舜得以考之何典，问诸何人，而为此邪？抑亦求诸其心一念之真知，权轻重之宜，不得已而为此邪？武之不葬而兴师，岂武之前已有不葬而兴师者为之准则，故武得以考之何典，问诸何人，而为此邪？抑亦求诸其心一念之良知，权轻重之宜，不得已而为此邪？使舜之心而非诚于为无后，武之心而非诚于为救民，则其不告而娶与不葬而兴师，乃不孝、不忠之大者。而后之人不务致其良知，以精察义理于此心感应酬酢之间，顾欲悬空讨论此等变常之事，执之以为制事之本，以求临事之无失，其亦远矣。其余数端，皆可类推，则古人致知之学，从可知矣。
来信说：「道的大端容易明白，所谓『良知良能，愚夫愚妇也能领会』。至于节目时变的详情、毫厘千里的谬误，必须学习然后才知。如今讲孝，谁不知道温凊定省？至于舜不告而娶、武王不葬而兴师、养志与养口、小杖与大杖、割股庐墓等事，处常与处变、过与不及之间，必须讨论是非，作为处事之本，然后心体无蔽、临事无失。」
「道之大端易于明白」，这话诚然。但后世学者忽视那容易明白的而不去遵循，却去求那难以明白的作为学问，这就是「道在近处却向远处求，事本容易却往难处求」。孟子说：「道就像大路一样，难道难以知晓吗？人的毛病在于不去走罢了。」良知良能，愚夫愚妇与圣人相同；只是圣人能致其良知，而愚夫愚妇不能致，这就是圣人与愚人的分野。节目时变，圣人岂有不知的？只是不专以此作为学问。他所学的，正是致其良知，以精审此心的天理，而与后世之学不同罢了。您不去致力于真知的致，却急急忙忙只担忧这些，这正是求难以明白者以为学的蒙蔽。
良知对于节目时变，就像规矩尺度对于方圆长短；节目时变不可预定，正如方圆长短不可穷尽。所以规矩果真立了，就不能拿方圆来欺它，而天下的方圆不可胜用了；尺度果真陈设了，就不能拿长短来欺它，而天下的长短不可胜用了；良知果真致了，就不能拿节目时变来欺它，而天下的节目时变不可胜应了。毫厘千里之谬，不在我心良知一念之微处省察，又能把学问用在哪里呢？这是不用规矩却要定天下的方圆，不用尺度却要尽天下的长短，我看只有乖张谬戾、日劳而无成罢了。
您说「讲孝于温凊定省，谁不知道」，然而能致其知的太少了。若说粗略知道温凊定省的仪节，便叫能致其知，那么凡是知道君当仁的，都可以叫能致其仁之知；知道臣当忠的，都可以叫能致其忠之知，那天下谁不是致知者呢？由此说来，致知必在于行、不行不可为致知，就很明白了。知行合一之体，不更加昭然了吗？
舜不告而娶，难道是舜之前已有不告而娶的人作了准则，所以舜能考之何典、问诸何人而这样做吗？还是求之于其心一念之良知，权衡轻重的合宜，不得已而这样做呢？武王不葬而兴师，难道是武王之前已有不告而兴师的准则，所以武王得以考之何典、问诸何人而这样做吗？还是求之于其心一念之良知，权衡轻重的合宜，不得已而这样做呢？假使舜之心不是真诚于不绝后嗣，武王之心不是真诚于救民，那么不告而娶与不葬而兴师，就是不孝不忠的大者。而后人不务致其良知，以在此心感应酬酢之间精察义理，却想悬空讨论这类反常之事，拿它作处事之本，以求临事无失，也就差得太远了。其余几端都可类推，那么古人致知之学，从此可以知道了。
Your letter says: 'The great outline of the Way is easy to see — innate knowledge and ability that ploughman and ploughwife can reach. But the details of circumstance and the hairspread-thousand-mile errors must be learned. Speak of filial piety: who does not know warming and inquiring? But Shun marrying without first telling his parents, King Wu taking the field before his father's burial, nourishing the will or the mouth, small rod or great, slicing the thigh, the mourning hut — constancy and crisis, excess and shortfall — these must be weighed for right and wrong as the touchstone of conduct, else the mind-substance stays veiled and affairs go wrong.'
That the great outline is easy to see — true. Yet later scholars slight what is easy to see and do not walk it, and study what is hard to see: the Way near, they seek it far; the task easy, they make it hard. Mencius: the Way is like a great road — is it hard to know? The sickness is not walking it. Innate knowledge and ability are one in ploughman and sage; only the sage can extend it, the ploughman cannot — there lies the divide. Details and changes — did the sage not know them? He simply did not make them his study. His study was precisely extending innate knowledge, refining the mind's heavenly principle — unlike later learning. You neglect the extending of true knowledge and fret anxiously over these — the very veil of studying the hard-to-see.
Innate knowledge toward details and changes is as compass and square toward circles and squares, the rule toward lengths: details cannot be fixed in advance any more than all circles and lengths can be enumerated. With compass and square truly set, none can deceive you about round and square, and the world's rounds and squares are inexhaustibly yours; with the rule laid down, none deceive you about length; with innate knowledge truly extended, none can deceive you about details and changes, and the world's details and changes are inexhaustibly met. The hairspread error — if it is not examined in the subtlety of a single thought of the mind's innate knowledge, where else would one's learning be applied? To fix the world's rounds without compass, exhaust its lengths without rule — I see only distortion and error, daily toil and no attainment.
You say: speak of filial piety in warming and inquiring — who does not know it? Yet those who extend that knowledge are few. If a rough notion of the ritual counted as extending knowledge, then all who know a ruler should be humane could be said to extend their knowledge of humaneness, all who know a minister should be loyal, of loyalty — who under heaven would not be extending knowledge? So seen, extension must lie in action, and without action there is no extension — and is not the unity of knowledge and action the plainer?
Did Shun, marrying unannounced, have a precedent to consult, a person to ask — or did he appeal to the true knowledge of a single thought in his mind, weigh the balance of weights, and act under necessity? Did King Wu, marching before burial, find a rule in some canon — or weigh the fitness in the innate knowledge of one thought and act under necessity? Had Shun's heart not been sincerely set on not cutting off the line, Wu's not sincerely on saving the people, then unannounced marriage and marching before burial would be the greatest of impieties and disloyalties. Later people, not extending their innate knowledge to examine rightness amid the mind's responses and exchanges, would hang in mid-air discussing such anomalies and clutch them as the touchstone of conduct, hoping to err nothing in the event — how far astray! The other cases follow by analogy, and the ancients' learning of extension may be known from this.