In the Lotus Sutra, there is a story of a child who playfully built a Buddhist stupa but attained Buddhahood by chanting Namu Buddha only once with a confused mind. This is called an incident of small virtue attaining Buddhahood. Tiantai considers this story as an important one since it candidly shows the Lotus Sutra's concept of the Truth of the One Vehicle. Tiantai thinks of behaviors of playfulness and confusion in mind as characteristics of human goodness, explaining the episode of small virtue attaining Buddhahood by the theory of Threefold Buddha-nature. The small virtue is Buddha-nature as conditional cause in Threefold Buddha-nature. Tiantai explains that Buddha-nature as complete cause also moves and Buddha-nature as direct cause appears by the movement of the Buddha-nature as conditional cause, attaining Buddhahood. This means that a practitioner is encouraged to arouse the resolve to attain Buddhahood on the basis of the Lotus Sutra.
