The biography is unusually eventful for a scholastic. He and his friend Uisang set out twice for Tang China. On the second attempt, according to the later accounts, they sheltered overnight in what they took for an earthen shelter, and Wonhyo drank water in the dark that refreshed him; in daylight he saw that the place was a tomb and the vessel a skull. The account has him concluding that since the same water was sweet in ignorance and foul in knowledge, the difference lay in the mind and not in the water, and turning back. Uisang went on and studied under Zhiyan.
That story is not in Wonhyo's writing. It appears in Chinese and Korean biographical collections from the tenth century onward, in differing versions, and its function is to explain why the greatest Silla thinker never studied abroad, which in that period required explaining.
What is better attested is the second half of his life. He fathered a son, Seol Chong, who became a major scholar of the Confucian classics and of the writing of Korean, and he ceased to be a monk in good standing. He then took his teaching to villages and markets, singing and dancing with a large gourd and using a phrase from a sutra as a refrain. The Samguk yusa says that by this means people who could not read came to know the name of the Buddha. A career that begins in the most technical scholasticism of its age and ends in a market square is not the usual shape, and Korean tradition has never treated the second half as a decline.