Within the Bagua the four elemental trigrams Heaven ☰, Earth ☷, Fire ☲, and Water ☵ have multiple meanings. The first two stand for Heaven and Earth, yin and yang. The trigram Heaven is made up of three yang lines and is pure yang, while the trigram Earth is made up of three yin lines and is pure yin. These trigrams Heaven ☰ and Earth ☷ also symbolize the breath, exhaled and inhaled.
Heaven ☰ is in command of bestowal while Earth ☷ is in command of receiving nourishment. Change refers to the change and transformation of yin and yang as they act upon each other creating the six trigrams Fire ☲, Water ☵, Thunder ☳, Lake ☱, Mountain ☶ and Wind ☴. While Heaven and Earth are seen as Father and Mother the other six trigrams are seen as Sons and Daughters.
Let us look at one son in particular.
☳ Trigram 4 is Thunder (zhèn or chen), the powerful voice of heaven. The trigram represents the arousing and shaking, what disturbs and unsettles, and therefore what triggers change. A force that can be destructive as well as constructive.
The trigram consists of two Yin (earthly) lines above a Yang (heavenly) one. So much earth on top of heaven can’t be stable. Also, when heaven is pressured by so much earth, it is bound to react. The combination of lines also suggests that this is a heavenly activity that affects earthly life — from its hiding, which is in a mysterious way.
The trigram’s Chinese character (above to the right) also means quake and tremor, as well as shake and excite.
Chen (Thunder) ☳ comes forth as an inkling,
— Wei Boyang, Cantong Qi
And the Yang Breath makes it start:
Initial nine, withdrawn dragon.
Hexagram 51 is named 震 (zhèn), “Shake”,”the arousing (shock, thunder)”; “thunder, excite, thrill, convulse, and tremor”. The advice is to maintain one’s concentration (“one did not lose the sacrificial wine in the ladle”). Its inner (lower) trigram is ☳ (震 zhèn) shake = (雷) thunder, and its outer (upper) trigram is identical.
Thunder is movement, action, represented as thunder. In the body of the hexagram, one yang is born below two yins, symbolizing the presence of movement within stillness. Here two thunders combine: From this action you arrive at that action, from that action you produce this action-a thousand actions, myriad acts, are all this one action; therefore it is called Thunder.
This hexagram represents acting so as to proceed on the Way, warily cultivating one`s original nature; it follows on the previous hexagram Revolution. Revolution involves gaining joy through illumination, refining metal by fire, removing old impurities. Removing old impurities does not mean forgetting things, forgetting the body, extinguishing the mind, and stopping thoughts; it is necessary to remove impurities in the midst of action in order to accomplish the task. This is because the Tao is alive, in movement; it is neither material nor void. We use worldly realities to practice the reality of the Tao, and use human affairs to cultivate celestial virtues; both action and non-action, comprehending essence and comprehending life, the endless work all must be done in the midst of activity. Therefore there is a path of development in Thunder.The coming of thunder means that within stillness there is suddenly movement; alarm followed by laughter means initial carefulness and subsequent ease. One moment of goodness in people’s minds, and the mind of Tao appears; this is heaven. One moment of evil in the mind, and the human mentality acts; this is hell. Hell and heaven do not exist in the world – they are both created in the human mind. It is the good and evil, falsehood and truth of thoughts, life and death which distinguish them. When you have thoughts, if you can practice self-awareness, gradually eliminating bad thoughts and gradually producing good thoughts, after a long time of consistency thought after thought all return to correct orientation, so that the mind of Tao is ever-present, and the human mentality does not arise: Filled with living energy, the real self sits peacefully in the center, tranquil and unperturbed, yet sensitive and effective.
Superior people cautiously practice introspection, striving to cause true mindfulness to grow and errant thought to vanish, not allowing the slightest defect to remain in the heart (square inch). This is because good and evil, fortune and calamity, depend on the movement of a single thought, which makes the difference between heaven and hell. One can only sustain good thoughts and get rid of bad thoughts if one is able to be cautious before the movement of thought and introspective after the movement of thought, guarding against dangers, aware of perils, not daring to slack off during the twelve double hours. Sustaining good thoughts and getting rid of bad thoughts until all bad thought is gone and all thought is purely good, ultimately wholesome without any ill, then thought may be active all the time without one being hindered by its activity, that activity returning to the state of total integration with the celestial design.
Liu Yiming, Hexagram #51 Thunder, The Taoist I Ching