Why read the old texts

Classical texts are not evidence about physiology. They are evidence about what careful people, over many centuries, noticed and valued, and about the practices they built around those observations. Reading them next to the biology of the previous chapters does two things. It shows which modern claims are old, and it shows that the tradition itself was often more sceptical, and more interested in attention, than its modern admirers.

The immortal who did not eat grain

The passage is a poetic image, and in the Zhuangzi it serves an argument about the limits of ordinary understanding, not a dietary rule. But “not eating the five grains” became the name of a practice. Bigu (辟谷), “avoiding grains”, appears in early Chinese sources on longevity alongside breathing exercises.

Historically, bigu rarely meant eating nothing. It meant avoiding cereals, often while eating other plant foods, medicinal substances or small amounts of other food, and it was combined with breathing and other exercises. It was a discipline of selection and regulation more than of total fasting.

A sceptic inside the tradition

The fourth-century alchemist Ge Hong, who believed firmly in the possibility of immortality, was nonetheless unimpressed by grain avoidance as a path to it.

Ge Hong’s line reads well next to Chapter 6’s trials. The tradition already distinguished between a practice that might be useful in a modest way and the extraordinary claims made for it.

Regularity, not extremity

The medical classic of the same tradition does not praise fasting at all. It praises measure.

“Eating and drinking had measure, rising and resting had regularity” is, in modern language, a statement about both quantity and timing: regular meals and a regular daily rhythm. It is close in spirit to the trials of Chapter 6 that found effects of when people eat, aligned with the body clock.

The Analects contains a related line among its descriptions of Confucius’s habits: “不时，不食”, “what is not in its time, he did not eat”. Commentators have read it in two ways: food out of season, or meals at the wrong time of day (杨伯峻 1980). Either reading is about timing, not about going without.

Not eating after noon

The Buddhist monastic rule forbids eating at the wrong time (vikāla-bhojana, 非时食), which means from midday until the following dawn. The rule binds fully ordained monks and nuns and novices, and lay Buddhists take it on as one of the eight precepts they observe on special days of observance (Buswell and Lopez 2014). In Chinese it is known as 过午不食, “not eating after noon”.

The rule was, in its origin, a matter of discipline and of the relations between monks and the lay people who fed them, not of health. In Chinese monasteries, where monks worked the fields in a cold climate, an evening meal was often allowed under the name yaoshi (药石), “medicine”, a compromise that shows the rule was lived with rather than applied blindly.

The fasting of the heart

The most important classical passage for this book is not about food at all. In the Zhuangzi, Confucius’s disciple Yan Hui says he is poor and has not drunk wine or eaten strong-flavoured food for months; is that not fasting? Confucius answers that this is the fasting of sacrifice, not the fasting of the heart.

The passage separates two things that the modern fasting movement tends to merge: abstaining from food, and the clearing of attention. Zhuangzi treats the first as a ritual preparation and the second as the real practice, which consists in “unifying the intent” and listening without grasping. Whatever one makes of its metaphysics, it is a precise description of a state of attention, and it does not depend on the stomach.

For this book’s question, the passage suggests a way to read both the classical claims and the modern ones. Some of the clarity people report while fasting may come from fasting’s biology. Some may come from the attention that a fast demands: the simplified routine, the removal of meals as events, the deliberate noticing of hunger without acting on it. Chapter 8’s log and Chapter 9’s trial both try to keep these apart.