The word for “empty” (sunna in Pali, shunya in Sanskrit) is used by the Buddha as a simple adjective to describe, for example, an empty room: “There are empty huts, monks. Meditate!” The same sense is extended to express the core Buddhist teaching of non-self: just as a room may be empty of furniture, the mind and body are “empty of self and of anything belonging to self.”
When the suffix -ta is added, the word takes the form of an abstract noun and is used to describe certain kinds of meditation. The Buddha tells his attendant monk Ananda in one of the two Discourses on Emptiness (Sunnata Sutta), “I often abide in emptiness,” and goes on to describe a meditation practice in which the objects of perception become gradually more subtle until one understands that “this field of perception is empty of the taints.” At this point “emptiness” becomes virtually a synonym of nibbana or nirvana, defined in the early texts as a mind devoid (empty) of greed, hatred, and delusion.
The word “emptiness” is best known for its central place in the Heart Sutra of the Mahayana tradition: “Form is emptiness; emptiness is form,” a phrase that is repeated also for the other four aggregates that construct our idea of self—feeling, perception, formations, and consciousness. The sutra goes on to say that “emptiness is the nature of all things,” thus expanding the psychological insight that a person is empty of self to the comprehensive metaphysical insight that all phenomena are empty of self-nature.
The 2nd-century philosopher Nagarjuna explains this with greater precision in his treatises by drawing out the implications of the teaching on impermanence and dependent origination. All things are in the perpetual process of arising and passing away, ever “becoming” and thus never actually “being.” Conditioned by multiple interdependent causes, all things are “empty” of any sort of independent or intrinsic nature and thus defy conceptualization.
As Buddhism moved into China, the notion of emptiness evolved in three directions: Along with the complementary idea of suchness (tathata), the Tiantai school gave it more substance by developing the concept of an underlying universal buddhanature; the Hwayan school, embracing both suchness and emptiness, stressed the thorough interpenetration of all phenomena; and syncretizing the Taoist idea of wu, as in wu-wei (non-action), the Chan (in China) and Zen (in Japan) schools encouraged the formless approach of wu-shin (no-mind, Japanese mushin), culminating in the seminal Japanese Zen koan Mu. Properly understood, things neither exist (since they vanish) nor don’t exist (since they occur); rather, they are simply empty—which calls for a nonconceptual intuition of wisdom, in Japanese kensho or satori.