Strengthening one’s will and mastering one’s mind before sleep can influence the quality of one’s dreams. Cheng Yi regarded “reflection on dreams” as an important contemplative practice for evaluating the results of one’s self-cultivation as demonstrated in the following two dialogues:

Someone asked, “Why do the things we don’t want to do during the day often appear in our dreams at night?” Cheng Yi replied, “It is simply because the heartmind is not settled (心不定). The things that people dream of are not limited to what they experienced during the day; they can be things that happened decades ago. The reason why these things appear in dreams is because they were already in the mind, and they were triggered by something that happened during the day or by something that affected the mind. Therefore, things that one dislikes during the day can also appear in dreams. It is like how the wind creates waves on the water; even after the wind stops, the waves continue to surge. If one has been well-preserved and nourished for a long time, one will not have such dreams. Sages and worthies do not have such dreams. For them, only omens and signs appear in dreams. Some people have clear vital-energy and no dreams, while others have muddled vital-energy and no dreams. A sage has no dreams because their energy is clear. If a person is extremely tired, they may not dream, but it is because their mind is clouded and blocked by muddled vital-energy. When Confucius dreamt about Duke Zhou,[43] it was different from an ordinary person’s dream. Therefore, people can evaluate the depth of their learning by reflecting on their dreams and quality of sleep. If they feel confused or disoriented about their dreams, it may indicate that their heartmind has not yet been settled, and their efforts to preserve and nourish themselves have not yet been solidified.”[44]

Someone asked, “The things that are tied to the heartmind will appear in dreams at night. If good things are tied to it, will they all be harmful when they appear in dreams at night?” Cheng Yi replied, “Even if it is a good thing, the heartmind can still be in agitation (心亦是動). When omens and signs enter the dream, they have no harmful effects, whereas everything else is just a result of delusional agitation.” Someone asked, “What about Confucius dreaming of Duke Zhou?” Cheng Yi replied, “This is a manifestation of a sage’s authenticity (誠). As a sage who sought to follow the Way of Duke Zhou, even during sleep, he did not forget about Duke Zhou. As he aged, he realized that the Way could not be realized, and he no longer dreamed of Duke Zhou. However, dreaming of Duke Zhou is not about delusionally conversing with him every night. The heartmind needs to be settled so that when one thinks, one thinks properly. Nowadays, people are driven by their hearts.” Someone then asked, “Who controls the heart?” Cheng Yi replied, “One can use their heartmind to control their heartmind. If one sets it free, the heartmind will roam aimlessly.”[45]

Cheng Yi distinguishes three types of dreams: First, dreams of omens and signs do not bring harm. Second, dreams of sages, such as Confucius, have no discrepancy from their true character in the daytime and hence demonstrate the authenticity of their personality, with no harm involved. Third, all other dreams, even those involving good things, are harmful. This is because the heartmind cannot control what happens in these dreams, and the occurrences often significantly differ from one’s waking state. Even if one dreams of good things, he or she cannot control their manifestation in the dream, making it an unreliable evaluation of his or her heartmind’s state. The first two types of dreams demonstrate the settledness of one’s heartmind and provide a proper evaluation of the state of the heartmind.

Cheng Yi’s belief that dreams of omens and signs indicate a settled state of the heartmind is particularly intriguing. He maintained that a heartmind that is in an authentic state allows for the ability of “psychological resonance” (感通), which can account for an individual’s ability to know things in an unusual, perhaps even psychic, manner. Cheng Yi explained,

Yang Ding’s belief in ghosts and gods boils down to the concept of psychological resonance. For instance, a person who has never learned to read in their entire life may fall ill one day and recite a whole poem by Du Fu (712–770 CE). This is made possible by the existence of pattern-principle. Everything in the world is either present or absent; what exists, exists, and what does not, does not. For example, Du Fu’s poetry truly exists in the world, so when someone’s heartmind reaches a state of concentration and unity (精一), they can resonate with the truth and gain knowledge of it. This is also why someone’s heartmind can visit the dream of another person in a different place; it is all due to the existence of pattern-principle that leads to psychological resonance of the heartmind. The dead can also appear in dreams for the same reason.[46]

What is psychologically resonated by the heartmind is nothing but the pattern-principle. The affairs of the world exist as they do, or they do not exist at all. This is true regardless of their occurrence in the past, present, or future. Dreams during sleep lack physical forms, but they are made possible by the existence of the pattern-principle. Shape and sound pertain to vital-energy. When things are born, vital-energy gathers, and when they die, it disperses and returns to nothingness. Sound requires the mouth, and touch requires the body. Once the material base (質) is destroyed, how can sound and touch exist? Therefore, without the pattern-principle, dreams cannot be trusted.[47]

According to Cheng Yi, the human ability of psychological resonance allows omens and signs to enter one’s dreams. The occurrence of such resonance depends on three factors: things must truly exist in the world, whether in the past, present, or future; the resonating person’s heartmind must be concentrated and unified and thus “settled”; and even if the material and Qi aspect of things disappears, the pattern-principle that explains their existence and coexistence with other things in the world still persists.[48] Thus, it is the existence of pattern-principles and their broad interconnection that is resonated by a pure heartmind, leading to the manifestation of omens and signs in dreams.

In his further analysis, Cheng Yi proposed that a pure heartmind, necessary for experiencing dreams of omens and signs, could be achieved through quiet-sitting meditation and other related contemplative practices. However, he cautioned against the pursuit of knowledge about future events as the primary goal of these practices as it contradicts the Ru commitment to moral self-cultivation and a rational lifestyle. While such knowledge may arise as an uncontrollable yet understandable result of contemplative practices, it should not be sought after for its own sake. The conversation that follows is prompted by a question about whether a Buddhist monk could really know in advance when a visitor is coming. Cheng Yi’s answer exemplifies this caution:

Someone asked, “Can a person living in seclusion know in advance when someone is coming to visit them?” Cheng Yi answered, “Yes, there are such people. For example, Dong Wu Jing of Songshan has this ability.” When asked how this was possible, Cheng Yi replied, “It is only through a quiet heartmind (心靜). A heartmind that has become quiet can reflect [like a mirror].” When asked whether a sage would be willing to do this, Cheng Yi answered, “Why do we need to involve sages and worthies in this? Even Buddhist monks who come close to understanding would not engage in such practices, let alone sages.”[49]