The Three Proofs or Sansho [三証 ] ; I. The Power of Threes

"In judging the relative merit of Buddhist doctrines,
I, Nichiren, believe that the best standards are those
of reason and documentary proof. And even more
valuable than reason and documentary proof is the
proof of actual fact." -- Nichiren Shonin

The Three Proofs, or 三証 {san sho) is a system that was used by Nichiren Shonin, to evaluate the wide diversity of conflicting Buddhist teachings, which were available in Japan, during the Kamakura Era. This was a burning issue, as various schools were competing for government patronage and secular power. As such, it was important to get the Dharma right, as the future of the nation was at stake. One thing, while the idea of the Three Proofs seems to be implicit in the Buddha's teachings, for example the Kalama Sutta, as far as I know, the concept was devised by Nichiren Shonin himself. I can not find any precise Pali or Sanskrit equivalent concepts. At any rate, the Three Proofs are Documentary Proof, Theoretical Proof, and Actual Proof.

Note: Due to length. I broke the original entry down into four (4).