The son tradition is said to have originated at the Dharma Assembly on Grdhrakuta-parvata (靈山會上; Vulture Peak) in India.
When the Buddha explained the Dharma to the one hundred trillion members of the Great Assembly, the Great Emperor of the First Heaven of the Realm of Forms (大梵天王) lowered a rainfall of flowers as an offering.

The Buddha raised a single flower and showed it to everyone.
It was only Kasyapa, the Buddha’s eldest disciple, who smiled radiantly in response.
At this the Buddha declared, “I submit my Treasury of the Eye of the Correct Dharma, Wondrous Mind of Nirvana, Formlessness of Forms, Entrance to the Subtle Dharma, Special Transmission Outside of the Teachings Without the Establishment of Words to Kasyapa.”

This event is referred to as “The Smile at the Presentation of the Flower to the Assembly on Grdhrakuta-parvata” (靈山會上 擧拈花示衆).

In another episode, when the Buddha was delivering a Dharma discourse at Pahuputraka (多子塔) in central India and Kasyapa had arrived late, the Buddha called him out to the front of the Great Assembly and shared his seat with him.
This event is referred to as “The Sharing of the Seat at Pahuputraka” (多子塔前 分半座).

And, finally, Kasyapa returned late to Kushinagara, having just completed his proselytization of the Dharma, to find that the Buddha had already passed away.
Kasyapa then circled the Buddha’s coffin three times and offered three prostrations at which the Buddha’s feet suddenly pushed out of the coffin.

This event is called “The Presentation of the Pair of Feet at the Coffin’s Edge Beneath the Pair of Sala Trees” (沙羅雙樹下 槨示雙趺).
These three events are called “The Three Places of Mind Transmission” in the son tradition.
They are the starting point for son.
In contrast, the speeches which the Buddha delivered across his lifetime are the basis for the doctrines of Buddhism.

This distinction between son and the Buddha’s teachings is reflected in the saying, “son is the mind of the Buddha, the doctrines are the words of the Buddha.”
In other words, son reveals the fundamental nature of reality without using words whereas the doctrines express that inexpressible reality through language.

Son and the doctrines reflect the same truth, but their methods, practices and traditions differ.
Thus, the Buddha is recognized as the common source of both son and the doctrines, but separate disciples are associated with the two traditions.

Kasyapa is representative of the path of meditation and Ananda, who reportedly memorized all of the Buddha’s discourses, is associated with the path of doctrinal study.