The Three Proofs; Documentary or Literary Evidence 文証

Documentary or Literary Proof [文証]

"ask them if there is documentary proof in the
teachings of any of the Buddhas"

-- Nichiren Shonin

Nichiren Shonin placed a lot of importance on recorded teachings or bunsho 文証; he was highly critical of the concept of 'transmissions outside the Sutras.' Of course, we now know that the Pali Suttas were composed well before thay were recorded; they were codified and subsequently passed down orally, for several centuries, before they were finally committed to writing. Moreover, the Mahayana Sutras, such as the Lotus Sutra, were composed in Sanskrit, centuries after the Buddha's passing. Some of the Mahayana Sutras were even likely composed in Chinese, and back translated into Sanskrit.

It is unclear if Nichiren was aware that the Buddha did not literally preach the Lotus Sutra per se. There are indications Nichiren did know that the Historical Buddha did not literally preach the 28 chapter Lotus Sutra as it was known to Nichiren and is still known to us. He mentioned that the original Lotus Sutra known in India was different than the Kumarijiva version he used. Nichiren also talked about different versions of the Lotus Sutra spoken by Buddhas and Bodhisattvas before Shakyamuni Buddha.

Moreover, the important point is not if the Lotus Sutra is a literal record of an actual event, but whether it teaches core Buddhist principles; does it concur with the Buddha's intent; does it speak with the Buddha's voice? Even in the Pali Canon, some Suttas were preached by the Buddha's disciples on his behalf. In these cases, the people who had heard the Sutta would sometimes later meet the Buddha, and relate what they had heard. The Buddha would then give his stamp of approval. For example, Dhammadinna the Nun taught the Culavedalla Sutta to a male lay follower named Visakha. That Sutta concludes with:

"Then Visakha the lay follower, delighting & rejoicing in what
"Dhammadinna the nun had said, bowed down to her and,
keeping her to his right, went to the Blessed One. On arrival,
having bowed down to the Blessed One, he sat to one side.
As he was sitting there he told the Blessed One the full extent
of the conversation he had had with Dhammadinna the nun.
When this was said, the Blessed One said to him, "Dhammadinna
the nun is wise, Visakha, a woman of great discernment. If you
had asked me those things, I would have answered you in the
same way she did. That is the meaning of those things. That is
how you should remember it." -- translated from the Pali by
Thanissaro Bhikkhu [link]

Of course, these days we can not just go ask the Buddha if a Sutra speaks with his voice. Instead, Nagarjuna [iirc] established a protocol called the Four Seals of the Dharma. These are; does a Sutra teach the Principles of anicca {impermanence}, anatta {no self}, dukkha {stress}, and Nirvana or cessation of stress? We can also check to see if it avoids the extreme views rejected by the Buddha; such as eternalism or annihilationism, Idealism or Materialism, hedonism or self mortification, and so on. If so, a Sutra passes the test of documentary or literal proof; and it can be said to speak with the Buddha's Voice.