The esoteric teachings, secret initiations and erotic imagery associated with Buddhist tantra have fueled no end of interest. But tantra may not be what you think it is.
What Is Tantra?
Countless practices of several Asian religions have been lumped together by western scholars under the heading "tantra." The only commonality among these practices is the use of ritual or sacramental action to channel divine energies. The earliest tantra probably grew out of the Hindu-Vedic tradition. Buddhist tantra developed independently of Hindu for many centuries, however, and they are barely related now in spite of a surface resemblance.
Even if we limit our study to Buddhist tantra, we are still looking at a vast range of practices and multiple definitions. Very broadly, most Buddhist tantra is a means to enlightenment through identity with tantric deities. It is sometimes also called "deity-yoga."
It's important to understand that these deities are not "believed in" as external spirits to be worshiped. Rather, they are archetypes representing the tantric practitioner's own deepest nature.
Mahayana and Vajrayana
One sometimes hears of three "yanas" (vehicles) of Buddhism -- Hinayana ("small vehicle"), Mahayana ("great vehicle") and Vajrayana ("diamond vehicle"), with tantra being the distinguishing feature of Vajrayana. Sorting the many schools and sects of Buddhism into these three categories is not helpful to understanding Buddhism, however. The Vajrayana sects are founded solidly on Mahayana philosophies and doctrines; tantra is a method by which the teachings are actualized. Vajrayana is best understood as an extension of Mahayana.
Further, although Buddhist tantra is most often associated with the Vajrayana sects of Tibetan Buddhism, it is by no means limited to Tibetan Buddhism. To a greater or lesser degree, elements of tantra can be found in many Mahayana schools, especially in Japan. Japanese Zen, Pure Land, Tendai and Nichiren Buddhism, for example, all have strong veins of tantra running through them. Japanese Shingon Buddhism is thoroughly tantric.
Origins of Buddhist Tantra
As with many other aspects of Buddhism, myth and history don't always find their way to the same ball park.
Vajrayana Buddhists say tantric practices were expounded by the historical Buddha. A king approached the Buddha and explained that his responsibilities did not allow him to abandon his people and become a monk. Yet in his privileged position he was surrounded by temptations and pleasures. How could he realize enlightenment? The Buddha responded by teaching the king tantric practices that would transform pleasures into transcendent realization.
Historians speculate that tantra was developed by Mahayana teachers in India very early in the first millennia CE, possibly as a way to reach those who weren't responding to teachings from the sutras.
Wherever it came from, by the 7th century CE tantric Buddhism was fully systemized in northern India. This was significant to the development of Tibetan Buddhism. The first Buddhist teachers in Tibet, beginning in the 8th century with the arrival of Padmasambhava, were tantric teachers from northern India.
By contrast, Buddhism reached China about the year 1. Mahayana Buddhist sects that emerged in China, such as Pure Land and Zen, also incorporate tantric practices, but these are not nearly as elaborate as in Tibetan tantra.
Sutra Versus Tantra
Vajrayana teachers compare what they call the gradual, causal or sutra path of Buddhism to the speedier tantra path. By "sutra" path, they mean following the Precepts, developing meditative concentration and studying sutras to develop seeds, or causes, of enlightenment. In this way enlightenment will be realized in the future. Tantra, on the other hand, is a means to bring this future result into the present moment by realizing oneself as an enlightened being.
The Pleasure Principle
I've already defined Buddhist tantra as "a means to enlightenment through identity with tantric deities." This is a definition that works for most tantric practices in Mahayana and Vajrayana.
Vajrayana Buddhism also defines tantra as a means to channel the energy of desire and transform the experience of pleasure into realization of enlightenment. According to the late Lama Thubten Yeshe,
"The same desirous energy that ordinarily propels us from one unsatisfactory situation is transmuted, through the alchemy of tantra, into a transcendental experience of bliss and wisdom. The practioner focuses the penetrating brilliance of this blissful wisdom so that it cuts like a laser beam through all false projections of this and that and pierces the very heart of reality." (Introduction to Tantra: A Vision of Totality , p. 37)
Behind Closed Doors
In Vajrayana Buddhism, the practitioner is initiated into incremental levels of esoteric teachings under the guidance of a guru. Upper-level rituals and teachings are not made public. This esotericism, combined with the sexual nature of much Vajrayana art, has led to much winking and nudging about upper-level tantra.
Vajrayana teachers say most of the practices of Buddhist tantra are not sexual, and what is mostly involves visualizations. Many tantric masters are celibate. It's likely nothing goes on in upper-level tantra that couldn't be shown to schoolchildren. I believe the reason for the secretiveness is that the teachings could easily be misunderstood or misused by people who are not being properly guided by an authentic teacher.