In the second century BC, a spiritual movement called Tantrism began to spread in India, significantly influencing Buddhism. Tantric Buddhism held that "nirvana" could be experienced through a series of physical acts engaging the senses. This contrasted sharply with traditional Buddhism, which defines nirvana as the transcendent state of freedom achieved by extinguishing physical desire.
The importance of the physical human body was central to Tantric beliefs. Rejecting the traditional ascetic character of earlier Buddhism, Tantric Buddhism promoted the full engagement of physical experiences. But the ambiguous language of Tantric texts made it unclear whether erotic terms were to be understood only symbolically, or expressed concretely in physical acts.
One example is yab-yum, a sexual ritual symbolizing the union of opposites. Though monks adhere to the rules of celibacy, images of divine couples engaging in a sexual embrace are relatively common in the memorial shrines ("stupas") and monasteries of Tantric Buddhists. Traditionally, however, yab-yum is not to be adopted freely by followers. "Only when one can blow a hole through a pile of barley flour with one's mind-power," goes an old adage, "is one able to engage in yab-yum." Moreover, only followers who have received proper instruction in the images' esoteric significance are allowed to view them.
Some scholars view Tantric Buddhism as the most evolved and enlightened statement of the original Buddha's teachings; others revile it as a denigration of those same teachings. Critics cite Tantrism's practices as barbaric expressions of popular spirituality, which oppose the Buddha's original cautions against sensual activities. The Tantric tradition is most widely known today as Tibetan Buddhism.