朱熹三十一歲後專意於儒學，但是早年沉潛於佛學，讀過十四種佛經，在其著作中所引用的佛家語，有六十一則，乃理學家之冠。佛學是朱子理學借鏡的思想範疇、思維方法。他藉闢佛來對比儒佛之異，自覺地開展建構了他的理學思想體係。朱子理一分殊的存有論係受華嚴理事圓融觀之啟發而借鏡資用。然而，朱子卻有出於佛學處，所謂：「佛說萬理俱空，吾儒萬理俱實。」 　　朱子的心性論採心統性情說。「性」屬本體論的「理」層級，「心」與「情」屬宇宙論的「氣」層級。他用佛家薰染的習心習性解釋氣質之性，以心之居敬窮理來變化氣質，轉惡向善說，頗有借鏡於佛家以智轉識去人欲之執。朱子謂人心道心乃一心之發，主張以道來端正人心，頗神似於《大乘起信論》一心開二門，以真常門轉心滅門的說法。朱子「敬」貫動靜及主靜坐養敬之實踐工夫，頗似禪定工夫。然而，朱子的「格物」非體證空理空性，而指向究明事物的所以然之理及人倫之間所當然法則。他對終極價值，抉擇了儒家生生不息的居仁由義之聖賢路而非涅槃寂靜之成佛，這是他出於佛學且有異於佛學處。

　　　　　Zhu Xi started concentrating on Confucianism after he was thirty-one years old. He spent most of his earlier years contemplating Buddhism. He read over fourteen Buddhist Canons. Zhu Xi used Buddhism to enhance his own “Li Xue” (Study of Principles). He used Buddhism as a method of reflection on his own philosophy. He primarily used Buddhism in order to contrast the difference between Confucianism and Buddhism. This in order to develop a more systematic structure for his own “Li Xue”. Zhu Xi's own ontology of Li (Being and beings) was borne out of the influence of the Avatamsaka Sutra. Upon reflecting on the differences between Confucianism and Buddhism. Zhu Xi contended that “Buddhism hold all principles to be empty, Confucianism holds all principles to be real.” Zhu Xi used a “Theroy of mind and nature (xing)” to systematize his notions of “xing” and “qing”. “Xing” is a part of “Li” in Zhu Zi's ontology, while mind (xin) and “qing” are part of “qi”. Zhu Xi employed Buddhist meditative practices in explaining his notion of “the temperament of nature (qi zhi zhi xing)”. A quiet, peaceful mind is needed in order to pass through “Li” to change one's character from evil tendencies toward good ones. Zhu Xi employed Buddhist notions in this process because they emphasized the lessening of desires. He tried to harmonize the mind of man with that of the mind of Dao. The mind of Dao would then elevate the mind of man. This is similar to a Mahayana notion of “one mind opening two doors”. Zhu Xi advocated a practice of “jin” which was a sitting in quietness, similar to Zen meditation. Zhu Xi's “investigation of beings” was not a proof of emptiness, like in Buddhism, but was rather an investigation into the “Li” of beings and how it could relate to human ethics. His ultimate value system was based on Confucian notions of “Ren” (humanity) and “Yi” (justice) as practiced by the “Sage” (sheng ren) and not the obtainment of Nirvana by a Buddha. Zhu Xi employed Buddhism in his philosophy, but he was not a Buddhist.