The above stories give us a glimpse of some of the important and unusual facets of Zen. But we have still to survey briefly the basic principles underlying the teaching, which cannot be understood intellectually without some training in Mahayana philosophy in general, and an adequate knowledge of the Hua Yen philosophy,  [Endnote 1-6] in particular.
In China Ch'an (Zen Buddhism) is also known as hsin tsung, meaning the Mind Doctrine, or "the teaching of Mind." This term is probably the best summary of all that Zen stands for, for what it teaches is the way to a full realization of Mind. Enlightenment is merely another name for the complete unfolding of the "inner" mind. Outside the deep and vast domain of Mind there is nothing to be enlightened about. Therefore, the sole aim of Zen is to enable one to understand, realize, and perfect his own mind. Mind is the subject matter and the keystone of Zen studies.
Buddhism and modern psychology both tell us that the mind has many "aspects and strata," of which some are of special interest to the field of psychology, and some to the fields of philosophy and religion. Zen, however, is interested not in these different "fields," but only in penetrating to the Essence, or the innermost core of the mind, for it holds that once this core is grasped, all else will become relatively insignificant, and crystal clear.
Before discussing this "inner core," let us see what Buddhism has to say about the general "features" of the mind. According to many Buddhist scholars, the simplest and most explicit way to delineate the "structure of mind" is to describe it as having three aspects or layers. The first aspect, or the "outer" layer, is the manifesting and active facet (Chinese: yung). This includes the active mental functions (of all the Eight Consciousnesses), [Endnote 1-7] both noetic and emotional, abstract and symbolic, such as love, hate, desire, reason, fantasy, memory, and so forth. This is the obvious aspect, of which every human being has had direct experience. It has been a primary study in the general field of psychology, but it is a subject in which Zen has little interest.
The second aspect, or "inner" layer, of the mind is called in Chinese hsiang, meaning "form" or "nature." Just what is this nature of mind? To put it succinctly, the nature of mind is self-awareness. To be self-aware means to be aware of the results of the play of consciousness, or to be conscious of the impressions received or the images captured by the consciousnesses. To be conscious of this play is an absolute, pure experience, in which there is no subject "knower" or object "known," the knower and the known having coalesced into one entity of "pure feeling." In this "pure feeling" there is no room for the dichotomy of dualism. Pure self-awareness is intrinsically and experientially non-dualistic, as the Buddhist sages and those of other religions have testified over many centuries. Self-awareness (the nature of mind) is not the function of knowing, but the knowing itself in its most intrinsic form. When one discovers this self-awareness, he finds his whole being changed. While engaging in any activity, he feels as though he were transcending the activity; he talks and walks, but he feels that his talking and walking is not the same as before – he now walks with an opened mind. He actually knows that it is he who is doing the walking; the director – himself – is sitting right in the center of his mind, controlling all his actions with spontaneity. He walks in bright awareness and with illumined spirit. In other words, the man who realizes self-awareness feels that he is no more the obedient servant of blind impulse, but is his own master. He then senses that ordinary people, blind to their innate, bright awareness, tread the streets like walking corpses!
If this self-awareness can be retained and cultivated, one will experience the illuminating aspect of the mind called by many mystics Pure Consciousness. When this illuminating consciousness is cultivated to its fullest extent, the whole universe is clearly seen to be in its embrace. Many mystics and Buddhists who have been misled consider this to be the highest state – the state of Nirvana, or the final stage of unification with the great Universal or "Cosmic" Consciousness. But, according to Zen, this state is still on the edge of Sangsara. Yogis who have reached this state are still bound by the deep-rooted monistic idea, unable to cut off the binding-cord of subtle clinging and release themselves for the "other shore" of perfect freedom. Therefore, though self-awareness, or its cultivated form – the illuminating consciousness – is a key to all inner realizations, basically and qualitatively it is still "clinging-bound." Buddhist Enlightenment is not gained through holding onto or inflating one's self-awareness. On the contrary, it is gained through killing or crushing any attachment to this illuminating consciousness; only by transcending it may one come to the innermost core of Mind – the perfectly free and thoroughly nonsubstantial illuminating-Voidness. This illuminating-Void character, empty yet dynamic, is the Essence (Chinese: ti) of the mind.
The important point here is that when the word "Essence" is mentioned, people immediately think of something quintessentially concrete; and when the word "Void" is mentioned, they automatically envision a dead and static "nothingness." Both of these conceptions miss the meanings of the Chinese word ti (Essence) and the Sanskrit word Sunyata (Voidness), and expose the limitation of the finite and one-sided way of human thinking. The ordinary way of thinking is to accept the idea that something is existent or nonexistent, but never that it is both existent and nonexistent at the same time. A is A or not A; but never is it both A and not A simultaneously. In the same way, the verdict of common sense on Voidness versus existence is: "Voidness is not existence, nor is existence Voidness." This pattern of reasoning, regarded as the correct and rational way of thinking, is advocated by logicians as a sine qua non and is accepted by common sense for all practical purposes. But Buddhism does not invariably follow this sine qua non, especially when it deals with the truth of Sunyata. It says: "Form does not differ from Voidness, and Voidness does not differ from Form; Form is Voidness and Voidness is Form." Buddhism also says that it is owing to Voidness that things can exist and, because of the very fact that things do exist, they must be Void. It emphasizes that Voidness and existence are complementary to each other and not in opposition to each other; they include and embrace, rather than exclude or negate each other. When ordinary sentient beings see an object, they see only its existent, not its void, aspect. But an enlightened being sees both aspects at the same time. This nondistinguishment, or "unification" as some people like to call it, of Voidness and existence, is the so-called Nonabiding Middle Way Doctrine of Mahayana Buddhism. Therefore, Voidness, as understood in Buddhism, is not something negative, nor does it mean absence or extinction. Voidness is simply a term denoting the nonsubstantial and nonself nature of beings, and a pointer indicating the state of absolute nonattachment and freedom.
Voidness is not easily explained. It is not definable or describable. As Zen Master Huai Jang has said, "Anything that I say will miss the point." Voidness cannot be described or expressed in words. This is because human language is created primarily to designate existent things and feelings; it is not adequate to express nonexistent things and feelings. To attempt to discuss Voidness within the limitations of a language confined by the pattern of existence is both futile and misleading. This is why the Zen Masters shout, cry, kick, and beat. For what else can they do to express this indescribable Voidness directly and without resorting to words?
The Buddhist teaching on Voidness is comprehensive and profound, and requires much study before it can be understood. This study is an essential preliminary to the understanding of Zen.
Returning to our original topic, the Essence, or the innermost core of the mind, we must first try to define it precisely. The Essence of mind is the Illuminating-Void Suchness. An enlightened Zen Buddhist not only knows the illuminating aspect of the consciousness but, most important of all, he also knows the void aspect of the mind. Illumination with attachment is decried by Zen as "dead water," [d] but illumination without attachment, or the Illuminating-Voidness, is praised as "the great life." The stanza [Endnote 1-8] which Shen Hsiu wrote to demonstrate his understanding of Zen to the Fifth Patriarch showed that he knew only the illuminating, not the void aspect of the mind.
When his "mirrorlike bright consciousness" stood against Hui Neng's "From the beginning not a thing exists!" it became so pitifully insignificant that it made him lose the race for the title of "The Sixth Patriarch of Zen." Hui Neng's "From the beginning not a thing exists!" expresses unmistakably the Essence of Mind as well as the innermost core of Zen. It was because of this deep understanding that Hui Neng won the title of the Sixth Patriarch.
"The body is the Bondhi tree,
The mind is like a clear mirror.
At all times we must strive to polish it.
And must not let the dust collect."
"Originally there is no tree of enlightnment,
Nor is there a stand with a clear mirror.
From the beginning not one thing exists;
Where, then, is a grain of dust to cling?" 
[3. Sutra of the Sixth Patriarch, Hui Heng, "The Platform Sutra": /zen/hui-neng/contents.htm ]
There are two interesting stories which illustrate the significance of realizing the void nature of one's own mind.
A. One day an angel, flying back to Heaven, saw below him a luxuriant forest enveloped in a great, glowing halo of light. Having traveled through the sky many, many times before, he naturally had seen numerous lakes, mountains, and forests, but had never paid much attention to them. Today, however, he noticed something different – a forest surrounded by a radiant aura, from which beams of light radiated to every part of the firmament. He reasoned to himself. "Ah, there must be an enlightened being in this wood! I shall go down and see who it is."
Upon landing, the angel saw a Bodhisattva sitting quietly under a tree absorbed in deep meditation. He thought to himself, "Now let me find out what meditation he is practicing." And he opened his heavenly eyes to see on what object or idea this yogi had focused his mind. Angels can usually read the minds of yogis, but in this case, much to his surprise, the angel could not find anything at all. He circled and circled the yogi, and finally went into Samadhi himself, but still could not find anything in the Bodhisattva's mind. Finally the angel transformed himself into a human being, circumambulated the yogi three times, prostrated himself, and said:
"I make obeisance to the Auspicious One;
I pay my homage to you,
O Lord of all sentient beings!
Please awake, come out of Samadhi,
And tell me upon what you were meditating.
With all my miraculous powers exhausted,
I still have failed to find out
What was in your mind."
The yogi smiled. Again the angel cried. "I make my obeisance to you, I pay homage to you! On what are you meditating?" The yogi merely continued to smile, and remained silent.
B. Hui Chung, [Nanyang Huizhong] who was Zen Master to the Emperor Su Tsung of the Tang Dynasty, was highly respected by the emperor, as well as by all the Zen Buddhists of China. One day a famous Indian monk named "Great Ear Tripitaka" arrived at the Capital. This monk was said to be able to read other people's minds without the slightest difficulty or hesitation.
Word of his accomplishments having reached the Emperor, the Indian monk was summoned to the royal palace to demonstrate his powers before Zen Master Hui Chung.
[The court and the people having assembled], Hui Chung asked Great Ear Tripitaka, "Do you truly have the power of reading others' minds?" "Yes, your Reverence, I have," he replied, and the following dialogue took place:
Hui Chung: "Tell me then, where does my mind go now?"
Great Ear Tripitaka: "Your Reverence is the Zen Master of a nation; how can you go to West Ssu Chuan to watch the boat races?" Hui Chung: "Tell me, where does my mind go now?" Great Ear Tripitaka: "Your Reverence is the Zen Master of a nation; how can you go to the bridge of Tien Ching to watch the monkeys at play?"
After a moment of silence, Hui Chung asked him, "Now, where does my mind go?"
This time Great Ear Tripitaka concentrated with intense effort for a long time, but he could not find anywhere a single thought of the Zen Master [and had to admit his failure]. Whereupon Hui Chung retorted, "You ghost of a wild fox! Where is your telepathic power now?"
Before concluding this discussion on "the three aspects of the mind," I must make one point very clear. This division of the mind into three "aspects" or "layers" should not be taken too literally, because, in fact, no such "aspects" or "layers" exist. Mind is one great Whole, without parts or divisions. The manifesting, illuminating, and nonsubstantial characteristics of Mind exist simultaneously and constantly – inseparable and indivisible in their totality. It is only for the sake of giving the reader a clearer comprehension of the matter that these "aspects" are introduced at all.