来书云：「周子曰『主静』，程子曰『动亦定，静亦定』，先生曰「定者心之本体」。是静定也，决非不睹不闻，无思无为之谓，必常知常存，常主于理之谓也。夫常知常存，常主于理，明是动也，已发也，何以谓之静？何以谓之本体？岂是静定也，又有以贯乎心之动静者邪？」
理无动者也。常知常存，常主于理，即不睹不闻、无思无为之谓也。不睹不闻、无思无为，非槁木死灰之谓也；睹闻思为一于理，而未尝有所睹闻思为，即是动而未尝动也。所谓「动亦定，静亦定」、「体用一原」者也。
来信说：「周敦颐说『主静』，程子说『动亦定，静亦定』，先生说『定者心之本体』。那么这个静定，决不是不睹不闻、无思无为的意思，必定是常知常存、常主于理的意思。常知常存、常主于理，明明是动、是已发，为什么叫作静？为什么叫作本体？莫非静定之外，还有个贯通心之动静的东西吗？」
理是不动的。常知常存、常主于理，就是不睹不闻、无思无为的意思。不睹不闻、无思无为，并不是槁木死灰；睹闻思为一于理，而未尝有所睹闻思为，就是动而未尝动。这就是所谓「动亦定，静亦定」「体用一原」。
Your letter says: 'Zhou said abide in stillness; Cheng said settled in movement, settled in stillness; you say settledness is the mind's substance. This still-settledness is surely not not-seeing, not-hearing, no-thought, no-doing; it must be ever-knowing, ever-preserving, ever-abiding in principle. Ever-knowing and abiding is plainly movement, already-aroused — why call it still? Why the original substance? Does something beyond stillness thread through the mind's movement and rest?'
Principle does not move. Ever-knowing, ever-preserving, ever-abiding in principle is precisely not-seeing, not-hearing, no-thought, no-doing. That is not withered wood and cold ashes: seeing, hearing, thinking, doing at one with principle, yet never really seeing, hearing, thinking, doing — moving, yet never moved. That is settled in movement, settled in stillness, substance and function one source.