Subhuti, someone might fill innumerable worlds with the seven treasures and give all away in gifts of alms, but if any good man or any good woman awakens the thought of Enlightenment and takes even four lines from this Discourse, reciting, using, receiving, retaining and spreading them abroad and explaining them for the benefit of others, it will be far more meritorious.
(DS, p. 74)
The Diamond Sutra is not strictly a Shingon Sutra. It is more associated with the Zen tradition. It does not come close in significance to the Mahavairocana Sutra or the Vajrasekara Sutra, as they are the foundation of Shingon and the basis of it's mandalas. It is not central like the Rishu-kyo's formative perspective on non-duality. The Diamond Sutra is not even as essential as the Heart Sutra, which Kobo Daishi Kukai asserted to contain all of the Buddhist teaching, both exoteric and esoteric.
That being said, the Prajna Paramita tradition and literature had an influential effect on both the Madhyamika and Yogachara schools of thought and practice. Kukai notes these as the sixth and seventh stages of spiritual development (KMW, p. 71-73) and as such, the foundation of the Mahayana. Additionally, Kukai makes an interesting, albeit passing reference to the Diamond Sutra in his, "The Meaning of Sound, Word and Reality":
In a sutra, therefore it is said: "[The Buddha is he] who speaks what is true and what is real, who tells things as they are, who utters neither deceitful words nor inconsistent words." These five kinds of speech are call mantra..."
(KMW, p. 241)
These connections to Shingon and the Prajna Paramita's place in the history of the development of Buddhist thought and practice make it worthy of exploration.
____________________________
A review of the basics and background of the Diamond Sutra is beyond the scope of this particular discussion. But that is not to suggest that it is not relevant or worthy of pursuit. For a workable starting point please review:
https://en.wikipedia.org/wiki/Diamond_Sutra
Also if one is unfamiliar with the chief disciples of the Buddha, basic information about Subhuti, a major figure in the sutra, can be found at:
https://en.wikipedia.org/wiki/Subh%C5%ABti
This discussion is specifically focused on select, and hopefully essential content within the sutra.
___________________________
As is typical of sutras, the opening begins by setting the scene and detailing those in attendance. In this sutra, Shakyamuni Buddha has just returned to his place of rest after his morning alms gathering and has taken his meal. Then he is approached by a senior disciple:
Now in the midst of the assembly was the Venerable Subhuti. Forthwith he arose, uncovered his right shoulder, knelt upon his right knee, and, respectfully raising his hands with palms joined, addressed the Buddha thus: World-honored One, it is most precious how mindful the Tathagata is of all the Bodhisattvas, protecting and instructing them so well! World-honored One, if good men and good women seek the Consummation of Incomparable Enlightenment, by what criteria should they abide and how should they control their thoughts?
(DS, p. 24)
Subhuti's question sets the context for the entire sutra and it is none other than the pursuit of complete awakening. This questions is what all that follows means to answer. And in classic style, the Buddha responds:
Very good Subhuti!... Now listen and take my words to heart.
(DS, p. 24)
In the following section the Buddha begins to build his response to Subhuti's essential question:
Subhuti, all Bodhisattva Heroes should discipline their thoughts as follows: All living creatures of whatever class, born from eggs, from wombs, from moisture, or by transformation, whether with form or without form, whether in a state of thinking or exempt from thought-necessity, or wholly beyond all though realms - all these are caused by Me to attain Unbounded Liberation Nirvana. Yet when vast, uncountable, immeasurable numbers of beings have thus been liberated, verily no being has been liberated.
(DS, p. 26)
Setting aside the questionable classification of all the types of beings in existence, here the Buddha essentially equates his awakening with a universal awakening of all sentient life. But then, just as quickly denies it. The reason for this contradiction quickly follows:
Why is this, Subhuti? It is because no Bodhisattva who is a real Bodhisattva cherishes the idea of an ego-entity, a personality, a being, or a separated individuality.
(DS, p. 26)
Here the Buddha teaches that the core mental discipline of the Bodhisattva path is not succumbing to the fallacy of self nature but instead embracing the reality of emptiness, or shunyata.
Here is the sutra's advice on how aspiring bodhisattvas should control their thoughts. Bodhisattvas are advised not to indulge in the error of the assumed reality of individual self-nature.
Later in the sutra, the Buddha elaborates on this, adding the quality of meditative discipline:
Therefore, Subhuti, all Bodhisattvas, lesser and great, should develop a pure, lucid mind, not depending upon sound, flavor, touch, odour or any quality. A Bodhisattva should develop a mind which alights upon no thing whatsoever...
(DS, p. 37)
This is how aspiring bodhisattva's should abide. Aspiring Bodhisattvas should strive to perceive without attachment, and if attachment arises, to recognize that they are necessarily
cherishing "the idea of an ego-entity, a personality, a being, or a separated individuality".
This pure lucid mind that alights on no thing whatsoever, can only be developed though meditation. And that is the core teaching of Ajari Tanaka - to develop a daily discipline of meditation.
The sutra ends with the penultimate statement, it's unambiguous core message. In this final section, the Buddha explains to Subhuti how the message of this sutra might be shared with others purposefully and with the intended effect - their fully awakening.
In closing, the Buddha says the following:
Thus shall ye think of all this fleeting world:
A star at dawn, a bubble in a stream;
A flash of lightening in a summer cloud,
A flickering lamp, a phantom, and a dream.
(DS, p. 74)
It is difficult to comment further... These words feels
neither deceitful nor inconsistent. It rings true and real. It mirrors things as they are.
In the hope that this is helpful and please enjoy the prajna paramita because if you dig in, there's a lot more.
jim
Bibliography
The Diamond Sutra and the Sutra of Hui Neng, Translated by A.F. Prince and Wong Mou-Lam, Shambhala Publications, 1969. Abbreviated as "DS".
Kukai: Major Works, Yoshito S. Hakeda, Columbia University Press, 1972. Abbreviated as "KMW".