A person's heart is the same as heaven and earth.
The blood circulating is similar to the moon and sun.
The eye must see all sides.
The ear must listen in all directions.
These four of the eight tomes of the kenpo gokui are singled out so as to show relationships between them in regards to referencing the Heaven, Earth, Sun, and Moon [eyes and ears].
Heaven = Head
Earth = Hara
Sun = Eyes
Moon = Ears
Notice that involving the body itself that the head is a symbol of Heaven since man stands on the earth and their head indicates a connection to the heavens above. The Earth is symbolized by the hara of the body since the hara is the center of the body where all energy and balance come to "one" point.
If the hara is strong then the body is anchored solidly to the Earth through the legs and feet. When we make reference to the sun and moon the I-Ching uses them to symbolize both the "eyes" and "ears."
The last two tomes also refer to how the eyes and ears are doorways to more than simply seeing and hearing. Through seeing and hearing along with mind training one increases intelligence and wisdom, both symbols of the sun, where this intelligence and wisdom provide a means for the practitioner to shed light on actions that are right and wrong allowing determination as to which is which so we can act accordingly.
The moon tells us that to truly listen is to sense danger or difficulty. It is symbolized by the gua of blood which could mean that to determine danger and difficulty we can use our eyes to see a method of overcoming in a manner best suited for each situation.
Use your head to train the mind in increasing intelligence and wisdom while centering all in the balance point of the hara clearing the meridians and energy so that the eyes and ears can see and actively listen in all directions increasing our awareness peripherally and spiritually.
Heaven is the force that initiates and becomes active in direct relation to the Earths receptive and passive posture which allows all things to be experienced in the moment. The balance achieved results in the ability to see and hear, sun and moon, for intelligent action and wisdom of knowing when and how to act.
The symbolism of heaven, earth, sun, and moon extend to the other tomes of the gokui, i.e. when talking of the eye seeing all sides or the ears listening in all directions. The four also symbolize the directions of north, south, east, and west.
When they are broken down into the separate trigrams we can also see the references to the other four directions, i.e. north-west, south-east, north-east, south-west. The sun symbolizes east, the moon west, Heaven north, and Earth south.
We can extrapolate from this the complexity of the kenpo gokui as it relates to our practice and further studies of the esoteric path we should balance our physical practice to reach "one" or equilibrium in the way.
If we also see the symbols of the sun we see the tortoise which has significance in the Chinese philosophy of the I-Ching. It shows that reference to a myriad of things is not literal but a means of expressing ideologies on life that go beyond our ability to express in mere words. Symbolism has its purpose and if we look beyond the lines and words of the gokui as well as the other classics we can discover so much more.