Cheng Hao and Cheng Yi shared a teacher, a household and a vocabulary, principle (li), material force (qi), humaneness (ren), and still arrived at two different philosophies, a divergence Chinese intellectual historians would spend the next eight centuries arguing over. Cheng Hao's own writing is sparser than his brother's, mostly recorded sayings rather than systematic treatises, and its center of gravity is a single passage: a person of ren regards heaven, earth and the ten thousand things as one body, feeling their pain as immediately as a numb limb fails to feel a needle.
That claim reads as psychology as much as metaphysics, an account of a certain kind of moral perception rather than a method for acquiring it, and it is exactly the emphasis Cheng Yi's own writing lacks. Where Cheng Yi built a program, the systematic investigation of things (gewu) that Zhu Xi would later canonize as orthodox method, Cheng Hao pointed instead at an immediate, already-present unity that only needs recognizing rather than constructing. Four centuries later, when Wang Yangming broke with the Cheng-Zhu reading of investigation as external study, he was in large part recovering Cheng Hao's older intuition: that principle is not out there to be gathered fact by fact, but already fully present in the mind that feels the ten thousand things as its own body.