In judging the relative merits of Buddhist doctrines, I Nichiren, believe that the best standards are those of reason and documentary proof. Even more valuable than reason and documentary proof is the proof of actual fact. Three Tripitaka Masters Pray for Rain The Writings of Nichiren Daishonin Vol. 1 p. 599
Nichiren Daishonin identifies three standards for evaluating the correctness of a Buddhist teaching:  documentary proof,  theoretical proof (the proof of reason), and  actual proof (the proof of actual fact). Documentary proof is what is recorded in the Buddhist sutras and other written sources, theoretical proof arises from evaluating and comparing Buddhist principles, and actual proof means what has been achieved by practicing and applying a teaching. Nichiren pursued all three proofs in investigating and determining which Buddhist teaching has the power to lead all people to enlightenment.
Founding Soka Gakkai president Tsunesaburo Makiguchi used the example of finding a good doctor to show how we might apply the principle of The Three Proofs in our daily lives. In treating an illness, we first find a doctor we can trust. A medical degree, diploma, and specialist credentials constitutes documentary proof. Next, we want to find out if the doctor’s diagnosis and proposed treatment are reasonable and accord with tested medical principles – theoretical proof. Last, we want to make sure the the doctor’s treatment for ourselves and their other patients results in a cure – actual proof. If not, we will have wasted our time, money and consequences that might even result in death.
In Buddhism, actual proof ultimately refers to the attainment of enlightenment, or Buddhahood, in this lifetime by an ordinary person. If we understand Buddhahood to mean a condition of indestructible happiness, rich in joy, wisdom, courage and compassion – then achieving actual proof means leading a genuinely happy and fulfilled life, a life of ultimate victory. Excerpted from The World Tribune Jan 16, 2009 “Got Proof?”