Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.
Having last month learned why Śākyamuni says ‘I shall pass away,’ we begin the Parable of the Skillful Physician and His Sick Children.
“Good men! All the Buddhas, all the Tathāgatas, do the same as I do. [They expound their teachings] for the purpose of saving all living beings. Therefore, [their teachings] are true, not false.
“I will tell you a parable. There was once an excellent and wise physician. He was good at dispensing medicines and curing diseases. He had many sons, numbering ten, twenty, or a hundred. [One day] he went to a remote country on business. After he left home, the sons took poison. The poison passed into their bodies, and the sons writhed in agony, rolling on the ground. At that time the father returned home. Some sons had already lost their right minds while the others still had not. All the sons saw their father in the distance and had great joy. They begged him on their knees, saying, ‘You came back safely. We were ignorant. We took poison by mistake. Cure us, and give us back our lives!’
“Seeing his sons suffering so much, the father consulted books of prescriptions, and collected good herbs. having a good color, smell and taste. He compounded a medicine by pounding and sieving the herbs, and gave it to them, saying, ‘This is a very good medicine. It has good color, smell and taste. Take it! It will remove the pain at once and you will not suffer any more.’
“The sons who had not lost their right minds saw that this good medicine had a good color and smell, took it at once, and were cured completely. But the sons who had already lost their right minds did not consent to take the medicine given to them, although they rejoiced at seeing their father come home and asked him to cure them, because they were so perverted that they did not believe that this medicine having a good color and smell had a good taste.
Nichiren discusses the sick children in his letter A Treatise Revealing the Spiritual Contemplation and the Most Verable One:
When we think of the Buddha’s intent reflected upon the clear mirror of these scriptures, we see that His appearance in this world was not for the sake of those who heard Him preach the Lotus Sūtra for eight years on Mt. Sacred Eagle. It was for those in the Ages of the True Dharma and the Semblance Dharma, and in the Latter Age of Degeneration. More precisely, it was not for the sake of those in the 2,000 years of the True Dharma and the Semblance Dharma, but for those like myself in the beginning of the Latter Age. The “sick ones” refer to the slanderers of the Lotus Sūtra after the death of Śākyamuni Buddha. It was “for those who did not perceive and accept this medicine excellent both in color and flavor” that the Buddha said he would “leave this excellent medicine.”
If we think of it this way, we can see why the bodhisattvas from underground did not appear during the Ages of the True Dharma and the Semblance Dharma. The 1,000-year Age of the True Dharma was appropriate for Hinayāna and provisional Mahāyāna Buddhism, but not fit for the preaching of the Lotus Sūtra in terms of both the “capacity” of those to be taught and the “time” for it to be preached. Therefore, four ranks of Bodhisattva-teachers (Four Reliances) preached the Hinayāna and provisional Mahāyāna teachings in order for the people to attain Buddhahood by nurturing the seed of Buddhahood that they had received during the lifetime of Śākyamuni Buddha. They did not preach the Lotus Sūtra then because they knew that if they had preached it, many people would have slandered it rendering it impossible to nurture the seed of Buddhahood. The capacity of the people for comprehension then was like that of those who listened to the Buddha preach in the first four of the five periods during His lifetime.
Kanjin Honzon-shō, A Treatise Revealing the Spiritual Contemplation and the Most Verable One, Writings of Nichiren Shōnin, Doctrine 2, Page 161