Emptiness in Theravada
There is a general misunderstanding that "emptiness" is a Mahayana teaching only, and not found in Theravada Buddhism.
Mahavagga of Samyutta Nikaya the Buddha says: “Emptiness (sunnata) is what I teach. A teaching that does not treat of emptiness is someone else’s teaching composed by some later disciple.”
Pancaka Nipata, Anguttara Nikaya says: "A discourse of any kind, though produced by a poet, or a learned man, versified, poetical, splendid, melodious in sound and syllable, is not an keeping with the teaching of the Buddha if not connected with emptiness (sunnata).”
Again the Buddha said: “And what bhikkhus is the path leading to the unconditioned? The emptiness concentratione signless concentratione undirected concentrations is called the path leading to the unconditioned.”
“Thus, bhikkhus, I have taught you the unconditioned. Whatever should be done by a compassionate teacher, out of compassion for his disciples, desiring their welfare, I have done for you. These are the roots of trees, bhikkhus, these are the empty huts. Meditate bhikkhus, do not be negligent, lest you regret it later. This is our instruction to you.”
The world is sunnata, empty.
Khuddaka Nikaya: “Because of being empty of self or of things due to self, it is consequently said that he world is empty.”
Emptiness means upekkha, equanimity, void of lusts, which Buddha emphasized.
Ajahn Buddhadasa of Suan Mokh said, “Don’t identify as ‘I’ or ‘mine’, act with clear awareness and there will be no suffering.”
“The sunnata of the Buddha means the absence of anything that we might have a right to grasp at and cling to as an abiding entity or self…The world is described as empty because there is nothing whatever that we might have a right to grasp at. We must cope with an empty world, with a mind that does not cling.”
“Nothing to do. Nothing to be. Nothing to have.”
The Dhammapada 92 says: “The arahants have emptiness (sunnata) and signlessness as their object.
Yasam sunnicayo natthi
Vimokkho yasa gocaro
Akase va sakuntanam
Gati tesam durannaya
They for whom there is no accumulation
Who reflect well over their food
Who have deliverance
Which is void and signless as their object…
Their course like that of birds in the air
Cannot be traced.
In Majjhima Nikaya 121, and also 122, the Buddha recommended a mode of perception that he called “entry into emptiness” in which one simply notes the presence or absence of phenomena, without making any further assumption about them; to look at “experience” or “process” without assuming “essence.”
“This world is supported by a polarity, that of existence and nonexistence. But when one sees origination of the world as it actually is with right discernment, ‘non-existence’ with reference to the world does not occur to one.
“The world is in bondage to attachments, clinging and baises. But one such as this does not get involved with or cling to these attachments, nor is he resolved on ‘myself.’ He has no uncertainty or doubt that mere stress, when arising, is arising; stress, when passing away, is passing away. In this, his knowledge is independent of others. It is to this extent that there is right view.” Samyutta Nikaya XII 15