The Chinese word for Xin can either refer to the heart or to the mind of a person. When ancient Chinese sages discussed humanity (ren) and cultivation, they invariably touched on this word, Xin. The difficulty arises if a translator wants to translate the word within a sentence not quite knowing if the sage actually meant the heart or the mind or even both. Complications can arise for students if they follow a wrongly translated meaning of the word by their favorite translator or various translators who do not have the requisite practice and/or deep understanding of what the sage meant.
Things that deal with the heart and mind are important matters to a human being and to students who earnestly practise. Following a wrong meaning or understanding can throw a practising student off the Path if not corrected. Take for example a student of meditation.
Did the sage meant to say empty the ‘heart’ or empty the ‘mind’ or both? Did he say still the ‘heart’ or the ‘mind’ or both?
To understand what the ancient sages really meant, we probably need to check with the writings of those who had attained the Way (the real teachers) and investigate. Since Buddhist and Daoist students in the West quite accept the popularity of Lu Dongbin, a Daoist heavenly immortal who also use the teachings of Buddha to teach neidan meditation, it may be appropriate to look into his Secret of the Golden Flower (Taiyi Jinhua Zongzhi) to discuss the understanding of the heart/mind, Xin:
On the heart : “This (lower fleshly) heart is dependent on the outside world. If a man does not eat for a day even, it feels extremely uncomfortable. If it hears something terrifying it throbs; if it hears something enraging it stops; if it is faced with death it becomes sad; if it sees something beautiful it is dazzled.” “One can make the heart move merely by running. Should one not be able to bring it to rest by concentrated quietness?”
On the mind : “When our ideas go very fast they imperceptibly pass into fantasies which are always accompanied by the drawing of a breath. Daily we draw innumerable breaths and have an equal number of fantasies.” “So then, should a man have no imaginings in the mind? One cannot be without imaginings. Should one not breathe? One cannot do without the breathing. The best way is to make a medicine of the illness. Since heart (mind) and breath are mutually dependent, the circulation of the light must be united with the rhythm of breathing.”
On the heart/mind : “While sitting, one must therefore always keep the heart quiet and the energy concentrated. How can the heart be made quiet? By the breath. Only the heart must be conscious of the flowing in and out of the breath; it must not be heard with the ears.”
In sitting meditation, we seek not to control the heart, lest it gets stifled, nor do we control thoughts since more thoughts will flow into the mind. Therefore both the heart and the mind focus on the breath (Qi). By and by, the heart is stilled and the mind emptied by the quietness and the breath.
If a neidan practitioner can quiet the heart/mind, still the heart, empty the mind, with Qi and the light and with the cultivation of bodily life, the spirit may one day manifest in the lighted room.
There is no necessity to believe what is written in this entry. Look up Xinzhai, the fasting (or emptying) of the mind, for the cleansing of the lighted room with Qi (translated as spirit) as indicated by Zhuangzi. Or look into the Neiyeh for what is written on Qi, Light, and the numinous (read spirit).
Empty the mind, still the heart said Laozi in TTC 16. Only an empty mind can be filled (TTC 22). Leihzi had advised not to restrain the mind, lest it gets injured. How could the mind be restrained if it is empty? The third line of Gen / Keeping Still in the Zhouyi advocates not to enforce quiet less the heart suffocates. The Qingjingjing (Classic of Purity) talks about achieving clarity, quietness, and stillness of the Xin.
If you still want to investigate further, research the emptiness of Heaven and the stillness of Earth in the Zhouyi. To truly understand the heart/mind (Xin) and to emulate heaven and earth is never easy and simple.