An empty mind is a balanced autonomic nervous system, which makes for an empty world. An empty world is, for example, the empty moon as it is in the empty sky as it is, with or without clouds. Empty reality is, to put it another way, a stone lantern in the garden or the sound of a temple bell, as it is.
Such was the Zen teaching of a Zen teacher who laboured long and hard to teach me. To understand emptiness in only this one way, however, as a state of the autonomic nervous system, would be a mistake. As a basis for translating Nāgārjuna, certainly, to insist that the Sanskrit śūnyatā always means balance of the autonomic nervous system, would be a mistake.
Still, if I threw the baby out with the bathwater, that would be my mistake.
So one meaning of emptiness, as the Buddha used the term, it is true, is a balanced mind, a mind that is free of polluting influences, a mind that is unencumbered by desire or lust or hatred or delusion or resentment (chando vā rāgo vā doso vā moho vā paṭighaṃ vāpi [see below]).
For my late teacher, Rev. Gudo Nishijima, the emptiness of a thus unencumbered mind is just a balanced autonomic nervous system, and a balanced autonomic nervous system is just the emptiness – “in the state of plus/minus zero” – of an unencumbered mind.
This meaning of emptiness, as the emptiness of an unencumbered mind, is implicit in the third discourse after the Buddha's full awakening, the Instruction about Burning. It is explicit in those suttas in the Majjhima Nikāya where the Buddha discusses suññatā, emptiness. One such sutta is Piṇḍapātapārisuddhisuttaṃ, the Discourse on Keeping the Almsround Pure (MN 151):
While the Buddha was dwelling at the Veḷuvana bamboo grove at Rājagṛha, there where the Glorious One was, in that direction (yena bhagavā tena....) approached Śariputra.
Ekamantaṃ nisinnaṃ kho āyasmantaṃ sāriputtaṃ bhagavā etad avoca:
Then the Glorious One said to venerable Śariputra, who was sitting to one side of him:
“Vippasannāni kho te, sāriputta, indriyāni, parisuddho chavivaṇṇo pariyodāto. Katamena kho tvaṃ, sāriputta, vihārena etarahi bahulaṃ viharasī?”ti
“Your senses, Śariputra, are clear; your complexion is pure and cleansed. In what place of recreation are you now fully dwelling?
“Suññatāvihārena kho ahaṃ, bhante, etarahi bahulaṃ viharāmī’’ti.
“I am now fully dwelling, Venerable Sir, in the abode of emptiness.”
‘‘Sādhu, sādhu, sāriputta! Mahāpurisavihārena kira tvaṃ, sāriputta, etarahi bahulaṃ viharasi. Mahāpurisavihāro eso, sāriputta, yad-idaṃ – suññatā.”
“Good, good, Śariputra! Truly, Śariputra, you are now fully dwelling in the abode of a great person. For this, Śariputra, is the abode of a great person – namely, emptiness.”
[The Buddha then teaches that a monk who wishes to abide in emptiness should ask himself, in regard to his almsround...]
atthi me tattha cakkhuviññeyyesu rūpesu chando vā rāgo vā doso vā moho vā paṭighaṃ vāpi cetaso’ti?
“Was there in my mind at that time desire or lust or hatred or delusion or resentment, in relation to forms discernible through the eye?”
“atthi nu kho me tattha sotaviññeyyesu saddesu…pe… ghānaviññeyyesu gandhesu… jivhāviññeyyesu rasesu … kāyaviññeyyesu phoṭṭhabbesu… manoviññeyyesu dhammesu chando vā rāgo vā doso vā moho vā paṭighaṃ vāpi cetaso?”ti
“Was there in my mind at that time desire or lust or hatred or delusion or resentment, in relation to sounds discernible through the ear... smells discernible through the nose... tactile stimuli discernible through the body.... proprioceptive experiences discernible through the inner sense ?”
“pahīnā nu kho me pañca kāmaguṇā?”ti
Are the five strands of sense-desire abandoned in me?
“pahīnā nu kho me pañca nīvaraṇā?”ti
“Are the five hindrances abandoned in me?”
“pariññātā nu kho me pañcupādānakkhandhā?”ti
“Have the five bodily aggregates of clinging been fully understood by me?”
“bhāvitā nu kho me cattāro satipaṭṭhānā?”ti
“Are the four ways of attending to mindfulness developed in me?”
“bhāvitā nu kho me cattāro sammappadhānā?”ti
“Are the four kinds of right effort developed in me?”
“bhāvitā nu kho me cattāro iddhipādā?”ti
“Are the four paths to power developed in me?”
“bhāvitāni nu kho me pañcindriyānī?”ti
“Are the five faculties developed in me?”
“bhāvitāni nu kho me pañca balānī?”ti
“Are the five strengths developed in me?”
‘bhāvitā nu kho me satta bojjhaṅgā”ti
“Are the seven factors of awakening developed in me?”
bhāvito nu kho me ariyo aṭṭhaṅgiko maggo”ti
“Is the noble eightfold path developed in me?”
bhāvitā nu kho me samatho ca vipassanā cā”ti
“Are quiet and insight developed in me?”
sacchikatā nu kho me vijjā ca vimutti cā”ti
“Have true understanding, and release, been realized by me in experience?”
Idam-avoca bhagavā. Attamano āyasmā sāriputto bhagavato bhāsitaṃ abhinandīti.
The Glorious One said this. The venerable Śariputra was uplifted and greatly rejoiced in what the Glorious One said.