Borrowing from Brahmanic religion, which taught that the universe had been created and destroyed over and over again throughout vast periods of time, early Buddhists believed that time flowed in immense cycles. Later Buddhism expanded this vision to include multiple universes, each with its own Buddha, and each universe subject to immense cycles of creation and destruction. Unless enlightened, one would be reborn again and again throughout all of these cycles.
This was samsara, the endless cycle of death and rebirth, and the ultimate goal of early Buddhism was to escape from samsara. The escape, called nirvana, was very different from an experience of sacred time such as one might expect in an ecstatic experience or shamanic journey. To attain nirvana was to extinguish any sense of self or individual experience; hence there was no sentient being remaining to experience a sacred time beyond ordinary time. There are two senses of nirvana: a this-worldly state when all attachments have been eliminated, and the non-state that occurs after death, when rebirth ceases.
The Buddha taught that those who occupied heavenly realms still existed in samsara, and even the gods were subject to rebirth. Even those human beings who were reborn in a Pure Land, although they were saved from future rebirths, were not free from samsara.
By the 2nd century C.E., Buddhist scholars had begun to use the Buddha's teachings about impermanence to deconstruct the doctrine of samsara. Mahayana sutras composed at around the same time revealed new teachings, purported to be from the Buddha, about a concept called shunyata, or "emptiness." In the Heart Sutra, for example, the Buddha was quoted as saying that when the bodhisattva Avalokitesvara realized that the five skandhas were all "empty," he "passed beyond suffering and difficulty." In other words, by understanding emptiness, he escaped samsara, just as the Buddha had done when he became enlightened.
This escape from samsara was very different, however, from that imagined by the early Buddhists. "There is no attainment whatsoever, because there is nothing to be attained," the Heart Sutra continued. "There is no ignorance and no ending of ignorance . . . no old age and death, no ending of old age and death." In just a few hundred words, the Heart Sutra deconstructed both the cycle of dependent arising and the path of the arhat seeking liberation from death and rebirth through the extinction of self.
The problem that led to this deconstruction was this: How did the Buddha continue to exist on earth and live the life of a human being after he was enlightened? Many solutions were offered to this problem. The basis of the Heart Sutra's solution was the concept of emptiness. This emptiness is not the equivalent of an absence of substance. Rather, everything is empty because nothing that exists has an "inherent nature" independent of other forms of existence.
One way to understand this is to think of a balloon. Before you blow it up, it is a piece of flexible material that can be stretched this way and that. Put some air in it and it becomes much larger, but it seems lighter; it almost seems to float as you bounce it around in the air. If you fill it with helium and do not tether it, it will indeed float away. Now prick the balloon with a sharp object and a loud sound will occur, while instantly the balloon takes on yet another form — usually multiple small, flat pieces that are similar in texture and flexibility to the first form, but with a different shape.
The "form" of the balloon was not a permanent, unchanging "form." In a matter of seconds, its form was changed several times. Placing "emptiness" into the balloon caused it to change "form," and removing the "emptiness" resulted in a different "form." The "emptiness" was not void of substance; it was composed of a substance that is not visible to the human eye.
Mountains are far more permanent than balloons and far more solid, but if you take a tiny piece of rock from a mountain and place it under a super high level of magnification, you will see that there is a lot of "empty" space inside it. Everything that exists is "empty" in this sense, and everything that exists also has "form." Within every form is emptiness, and within all emptiness, form.