Zazen as No-thought and Non-thinking

Zazen simply means seated meditation. What type of meditation happens while seated can vary. First of all, to make it a Buddhist meditation (sammāsamādhi - right immersion), other elements of the path have to be present as well.

'And what is noble right immersion with its vital conditions and its prerequisites? They are: right view, right thought, right speech, right action, right livelihood, right effort, and right mindfulness. Unification of mind with these seven factors as prerequisites is called noble right immersion with its vital conditions and also with its prerequisites.'
(MN 117)

In case of the bodhisattva path it is similar (MPPS XLV.2.III), for meditation (dhyāna) to be a perfection (pāramitā) it requires the perfection of wisdom (prajñāpāramitā).

The Zen view on the connection between meditation (samādhi) and wisdom (prajñā) is that they are inseparable:

'Meditation and wisdom are of one essence, not different. Meditation is the essence of wisdom, and wisdom is the function of meditation. At times of wisdom, meditation exists in that wisdom; at times of meditation, wisdom exists in that meditation.'
(Platform Sutra, ch 4, BDK ed, p 41-42; T48n2008p352c14-16)

And in the same chapter it is written:

'Good friends, there are also those who teach meditation [in terms of] viewing the mind, contemplating tranquility, motionlessness, and nonactivation. You are supposed to make an effort on the basis of these. These deluded people do not understand, and in their grasping become mixed up like all of you here. You should understand that such superficial teachings are greatly mistaken!'
(p 43; p353a5-7)

The way it is put into practice has various names, one common label is nonthought/no-thought/non-thinking (wunian 無念). Again, from the Platform Sutra:

'Good friends, in wisdom’s contemplation both interior and exterior are clearly penetrated, and one recognizes one’s own fundamental mind. If you recognize your fundamental mind, this is the fundamental emancipation. And if you attain emancipation, this is the samādhi of prajñā, this is nonthought.
What is nonthought? If in seeing all the dharmas, the mind is not defiled or attached, this is nonthought. [The mind’s] functioning pervades all locations, yet it is not attached to all the locations. Just purify the fundamental mind, causing the six consciousnesses to emerge from the six [sensory] gates, [causing one to be] without defilement or heterogeneity within the six types of sensory data (literally, the “six dusts”), autonomous in the coming and going [of mental phenomena], one’s penetrating function without stagnation. This is the samādhi of prajñā, the autonomous emancipation. This is called the practice of nonthought.
If one does not think of the hundred things in order to cause thought to be eradicated, this is bondage within the Dharma. This is called an extreme view.
Good friends, to be enlightened to the Dharma of nonthought is for the myriad dharmas to be completely penetrated. To be enlightened to the Dharma of nonthought is to see the realms of [all] the buddhas. To be enlightened to the Dharma of nonthought is to arrive at the stage of buddhahood.'
(ch 2, p 33; p351a25-b5)

This was summarised by Guifeng Zongmi in the following words:

'When you are aware that all characteristics are void, it is true mind, no mindfulness (wunian 無念). If a thought arises, be aware of it (nian qi ji jue 念起即覺); once you are aware of it, it will disappear. The excellent gate of practice lies here alone.'
(Chan Letter, in Zongmi on Chan, p 88; T48n2015p403a4-6)

The main instruction is included in The Manual of Zazen Practice by Changlu Zongze:

'One must not delve into thoughts (siliang 思量) that arise, whether good or bad. As soon as a thought arises, he must become aware of it (nian qi ji jue 念起即覺). He must always be aware of what arises in the sphere of consciousness without losing clear discrimination, and without becoming dull or scattered. A myriad of years is nothing but one moment of thought, which is neither discontinuous nor continuous. This is the essential Way (or method) of Zen practice. Zazen is in itself the doctrinal gateway of “comfort and ease.” (anle famen 安樂法門)'
(The Baizhang Zen Monastic Regulations, BDK ed, p 256; T48n2025p1143a12-15)

Not delving into good or bad thoughts is related to the instruction given to Huiming by Huineng:

'Do not think of good (bu si shan 不思善), and do not think of evil (bu si e 不思惡). At just such a time, what is Elder Huiming’s original face?'
(Platform Sutra, ch 1, BDK ed, p 25; T48n2008p349b24-25)

So, based on Zongze's manual Dogen wrote his zazen instructions with a little twist:

'Sitting in balance in the mountain-still state (kotsukotsu 兀兀), think (shiryō 思量) the concrete state of not thinking (fushiryō 不思量). How can the state of not thinking be thought? It is non-thinking (hishiryō 非思量). This is the real secret of zazen. Sitting in zazen is not learning Zen meditation. It is the great peaceful and joyful (anraku 安樂) gate of Dharma (hōmon 法門). It is untainted practice and experience.'
(Zazengi, in SBGZ, BDK ed, vol 3, p 226; T2582_.82.0217c08-13)

In the first part Dogen follows a story about Yaoshan Weiyan:

Once when the master was sitting, there was a monk who asked,
‘Pondering (siliang 思量) what so deeply (wuwu 兀兀)?’
‘Pondering an imponderable (busiliang 不思量),’ replied the master.
‘How to ponder an imponderable?’ asked the monk.
‘It is not something ponderable (feisiliang 非思量),’ said the master.
(Records of the Transmission of the Lamp, vol 4, 14.335; T51n2076p311c26-28)

That's how the term no-thought (wunian 無念) became non-thinking (hishiryō 非思量) that is used in Soto Zen for the optimal mental state during zazen. Non-thinking is an interesting choice also because it connects zazen to the realisation of the buddhas as stated in the Lotus Sutra:

'The real intention of all the buddhas in adapting their explanations to what is appropriate is difficult to understand. Why is this? Because I have expounded the teachings with innumerable skillful means and various kinds of explanations and illustrations. Yet this Dharma is beyond reason and discernment (hi-shiryō-funbetsu 非思量分別). Only the buddhas can understand it.'
(Lotus Sutra, ch 2, BDK ed, p 30; T9n262p7a18-21)

And it is also an expression that appears in Sengcan's poem:

'Nothing to linger upon,
Nothing to remember.
Clear, empty, and self-illuminating,
The mind exerts no effort.
This is beyond the sphere of thought (feisiliang 非思量),
Which reason and feeling cannot fathom.
In the Dharma Realm of True Suchness,
There are neither self nor others.
To reach accord with it at once
Just practice non-duality.
Non-duality embodies all things,
As all things are inseparable.'
(Trust in Mind, in A Collection of Selected Buddhist Texts, p 85-86; T48n2010p376c28-377a3)

Daily Karma

There is a way to comprehend karma in a rather tangible way. It means the cycle of view - intention - action - experience - view - ... That is, one has a set of values and preferences that define what one wants or doesn't want, based on that one chooses to act one way or another. The consequences - closer and later in time - feel good or bad, agreeable or disagreeable, based on one's interpretation (view) and also giving a feedback on whether one's choice of action was a good or a bad idea and that way reinforcing or weakening one's original view. Then based on previous experience that is modified by one's understanding of that experience one makes new choices. That's in brief the cycle of samsara on a daily level. As long as one is governed by the fundamental wrong view of trying to ascertain, obtain, or maintain an agreeable state of existence, there is suffering. Seeing that this cycle is nothing more than a series of processes not controlled by anyone, that is recognising its empty nature, and that way relinquishing the recurring craving for the imagined agreeable state, thus obtaining peace and the cessation of the cycle.

Twelve Links of Dependent Origination

When a mind (vijñāna) conditioned (saṃskāra) by ignorance (avidyā) conceives (nāmarūpa) an impression (sparśa) through the six senses (ṣaḍāyatana) that feels (vedanā) pleasant/unpleasant, then it craves (tṛṣṇā) for its continuation/discontinuation, attempts to keep (upādāna) and identify (bhava) with it, but with its birth (jāti) comes its decay and demise (jarāmaraṇa).

When one is ignorant about how suffering arises and ceases, then one concocts/fabricates physical, verbal, and mental activities to perform, and that results in a mindset/attitude (consciousness) that regulates one's bodily and mental functions, thus colouring one's senses, so when there is an impression (contact) and a related quality (feeling), then necessarily one enjoys and delights in it (craving), therefore wants to hang on to it (clinging), and that turns into an identity (becoming), so it defines one's whole being (birth), but eventually, like everything else, it'll fall apart and thus cause pain.

The twelve links are present at each instance of suffering ignorantly. It's not that when there is one link present, like feeling, then the others are missing. When one feels something pleasant (note that already that single feeling exists in a compounded way within a network of conditions), then there had to be a contact with something through the six sense gates, and that thing is recognised as a particular form with a particular name, and one has an opinion (mindset) of that thing that is primarily driven by an intention, and that motivation can exist because of not knowing any better. Because of the presence of the preconditions it is inevitable that the feeling of something pleasant is met with liking it, the need to hold it, and thus defining one's whole experience of the world, what in turn gives rise to a fixed set of particular behaviour and mentality that will necessarily deteriorate and decease.
Let's take an apple juice as an example for a pleasant object. To recognise it as such one needs to come in contact with it (seeing/tasting/imagining), and that contact to be meaningful one needs a couple of preconditions, like knowing what an apple juice is and an opinion of it whether one likes it or not. Since one recognises the apple juice as something desirable, it is a pleasant object. With the recognition of the tasty juice comes the thirst for it, the thirst develops into the need to have it, that grows into the thought of being the one who delightedly experiences drinking it. With those present arises the view and entity of the subject partaking of an object, the subject being the actor and enjoyer, while the object what is seen/tasted/imagined. Then with the changing of the object, for instance finishing a glass of apple juice, the subject loses its reason to exist and experiences some dissatisfaction because of that. All this can go down in a few seconds from seeing the apple juice to having drunk it. And then it happens again and again with the various experiences happening, the previous conditioning the next.

Chan by Investigating Body and Mind

 “You only have to carefully look inside of yourself (反觀) and examine skandhas, āyatanas and dhātus one by one: is there a tiniest thing to be obtained?” 

[The guest] replied: “I have carefully contemplated it, and have not seen anything which can be obtained.”

 The Master said: “Have you destroyed the characteristic of the mind and body?” 

[The guest] answered: “The nature of the mind and body is such that they disappear by themselves, what is there left to destroy?” 

The Master asked: “Is there any thing outside the mind and body?” 

[The guest] answered: “The mind and the body have no outside, how could there be any thing?” 

The Master asked: “Have you destroyed the mundane characteristic?” 

[The guest] answered: “The mundane characteristic is the absence of characteristics, what is left there to destroy?” 

The Master said: “If so, than you have overcome the mistake.”

(source: The Chán Teaching of Nányáng Huìzhōng (-775) in Tangut Translation by Kirill Solonin; also found in Records of the Transmission of the Lamp, vol 7, 28.1; T51n2076p438c26-439a2)

Will All Attain Enlightenment?

In the Saccasaṁyutta there are a series of suttas (SN 56.61-131) on the rarity of the precious human birth, like how few are born as humans (SN 56.61; see also the blind turtle simile: SN 56.47-48), not in a borderland (SN 56.62), and with wisdom (SN 56.63), and among those passing away as humans many are reborn in the lower realms and few in the heavens (SN 56.102-107). On the other hand, attaining insight is supremely difficult (SN 56.45) but without it there is no liberation (SN 56.44). At the same time, speculating about various matters of the world is not beneficial for the path, rather one should get busy with understanding the Dharma (SN 56.41).

The Buddha was asked (AN 10.95): 'when Master Gotama teaches in this way, is the whole world saved, or half, or a third?'
And Ananda explained: 'it’s not the Realized One’s concern whether the whole world is saved by this, or half, or a third. But the Realized One knows that whoever is saved from the world—whether in the past, the future, or the present—all have given up the five hindrances, corruptions of the heart that weaken wisdom. They have firmly established their mind in the four kinds of mindfulness meditation. And they have truly developed the seven awakening factors. That’s how they’re saved from the world, in the past, future, or present.'

From No Mindfulness to No Mind

Zen mindfulness is principally no mindfulness (wunian 無念). If that is still too obscure then one can always watch a phrase (kanhua 看話), i.e. a word head (huatou 話頭). In case that proves to be difficult for the moment, it is best to be mindful of the Buddha (nianfo 念佛) as much as possible. That can lead to asking 'Who is mindful of the Buddha?' (nianfo shi shei 念佛是誰), and with that may come the realisation of no mind (wuxin 無心).

True Mind is No Mind

'Worldly wealth is subject to loss, theft, damage, decomposition, and a thousand other things. But nothing ─ not time, not natural forces, not human beings ─ can take away Dharma wealth. However, if your mind moves, the wealth is gone. Likewise, the instant your mind stops moving, you enter the door of Ch'an. And at that moment, your mind is pure; that is, there is no mind. At that moment, you instantly realize the wisdom of non-arising. Such wisdom is not subject to the cycle of birth and death.'
(The Sword of Wisdom by Sheng-yen, online p 152)

'What is called no-mind is nothing other than a mind free from deluded thought.'
(Treatise on No-Mind, 17; T85, no. 2831, p. 1270a15)

'We have to come to a clear understanding as to the true nature and Absolute Existence of Dharma. This truth is that “as all phenomena are strictly dependent, conditioned, and relative, they are therefore impermanent.” This is the Absolute Existence of Dharma. If we have such a penetrative understanding of the nature of phenomena, of the notion of emptiness, we will enter the gateway of Chan, or align ourselves with the Tao. This is what Bodhidharma’s first formula, “facing a wall is the way to pacify the Mind”, teaches us.'
(Gates of Chan (入禅之门), p 33)

Absolute Existence is the translation of 实相 (實相 tattvasya-lakṣaṇam), defined in MMK 18.9:
"To know for oneself, not following others,
Calm extinction, without sophistries,
No differences and no distinctions;
This is termed the 'true character'."
(T30, no. 1564, p. 24a7-8; tr Bocking)

The quote (“缘生无自性、无自性故空”) is from the commentary to MMK 7.17:
'dharmas arising from causal conditions have no self-nature, and having no self-nature are empty'
(T30, no. 1564, p. 10c20-21, tr Bocking)

'But what is this “nature”? If we are to use words and concepts, we can call it the buddha-nature, which is our original nature. This is the nature of awakening, the nature of emptiness or selflessness. Emptiness means relationships, connectedness. Because all things are connected, nothing is fixed, and everything is free.'
(The Essence of Chan by Guo Gu, 1.2)

"But how could one [even] gain the ability to know that it is no-mind [that sees, hears, feels, and knows]?"
"Just try to find out in every detail: What appearance does mind have? And if it can be apprehended: is [what is apprehended] mind or not? Is [mind] inside or outside, or somewhere in between? As long as one looks for mind in these three locations, one's search will end in failure. Indeed, searching it anywhere will end in failure. That's exactly why it is known as no-mind."
(Treatise on No-Mind, 6; T85, no. 2831, p. 1269b9-13)

'it is possible to directly comprehend a thing’s nature and its Absolute Existence, that is, we can know this teacup as the combination of all its necessary conditions. After stripping all those conditions away one by one, we may ask: “Where then is the teacup?” It has no self-identity; it is simply a combination of causative conditions. In this way we can see through to the absolute nature of the thing. Such insight is wisdom. What we observe when looking with such an insight is “suchness” itself, the true actuality, the ultimate truth and Absolute Existence.'
(Gates of Chan, p 146)

'Emptiness is relationships, which means that there is no separation between self and other. Instead of seeing ourselves in the center of everything, we see the connectedness of everything. Selflessness is one of the core teachings that free us from attachments and suffering. It is the realization that all things— including us—are related, that they have no permanent, separate, independent self. This does not mean that we have no personality; we always have one, even after enlightenment. It is not that we do not know our name, or do not recognize anyone, or that our mind is blank—enlightenment is not that at all. What is absent in the enlightenment of selflessness is the fundamental assumption of a permanent, separate, independent self within us and within all things. What we consider as separate, independent, permanent, and solid is actually the coming together of everything else.'
(The Essence of Chan by Guo Gu, 2.4)

'Question. – It is quite possible that the saṃskṛtadharmas, coming from the complex of causes and conditions (hetupratyayasāmagrī), are without intrinsic nature (niḥsvabhāva) and therefore empty (śūnya). But the asaṃskṛtadharmas, which are themselves not dharmas coming from causes and conditions, are indestructible (akṣaya), inalterable (abhedya), eternal (nitya) and like space (ākāśasama). How would they be empty?
Answer. – As I have just said, outside of the saṃskṛtas, there are no asaṃskṛtas, and the true nature (bhūtalakṣaṇa) of the saṃskṛtas is exactly asaṃskṛta. The saṃskṛtas being empty, etc., the asaṃskṛtas themselves also are empty, for the two things are not different.
Besides, some people, hearing about the defects of the saṃskṛtadharmas, become attached (abhiniveśante) to the asaṃskṛtadharmas and, as a result of this attachment, develop fetters.'
(MPPS 48.8; T25, no. 1509, p. 289b5-11)

'All vexations of body and mind are wandering thoughts. Today someone recited a line from the Diamond Sutra, "Not abiding, the mind arises," and asked if it meant having no motivation, or doing nothing. Actually, the line describes the state after enlightenment when wisdom arises and the mind is not attached to anything. When you have motivated yourself to meditate, the will to practice is still a wandering thought. When we are practicing hard, we are still using this false mind (wandering thoughts). On the other hand, true mind is no-mind. How can one practice if there is no-mind?
Everything you do, including meditation, falls under false mind, but if you don't use this false mind to practice, you will never arrive at true mind. The progression of practice is from scattered mind to concentrated mind, from that to unified mind, and from one-mind to no-mind. The first three stages - scattered, concentrated, and unified mind - are all illusory. Only rarely can someone realize no-mind without first going through these three stages.'
(Song of Mind by Sheng-yen, 6.1)

'If you cut off the past and future, and discover that the present does not exist either, at that moment there is no mind. This itself is becoming a Buddha. But if, in the next instant, mind, past, future and present return, then you are again an ordinary sentient being.'
(The Sword of Wisdom by Sheng-yen, online p 187)

'It is not perishable and cannot be praised,
Its substance is like limitless space.
Without leaving where it is, it is constantly clear.
When seeking, you know it cannot be found.
It cannot be grasped, nor can it be discarded;
It is obtained only in the unobtainable.

It is impossible to attain Buddha-nature by grasping for it. Buddha-nature is not something that can be explained or praised. It is as big as empty space and as small as empty space. It contains everything. In the quickest snap of one's fingers every moment of time is present within it. How can one possibly praise something that is beyond comprehension?
No matter where you are, it is possible to perceive Buddha-nature. But the instant you chase after it, it is gone; you will not be able to find it.
Buddha-nature can neither be grasped nor discarded. Only when it is unobtainable can it be obtained. As you can imagine, there is not much one can say about this. So if you cannot attain it, how do you attain it? How can you attain something by not attaining it? In truth, there is no attainment. Buddha-nature is here right from the start.
My question is, "Where is your mind?"
I will tell you something about Buddha-nature: When there is no mind, that is it. But when you put your mind on Buddha-nature, it's gone.'
(The Sword of Wisdom by Sheng-yen, online p 165)