In explaining the difficulty to be born a human being and to encounter the teaching of the Buddha, the Nirvana Sūtra, fascicle 33, states:

“The Buddha then picked up a small portion of soil placing it on a fingernail and asked Kāśyapa whether or not this was more than the soil in the worlds all over the universe. Kāśyapa Bodhisattva answered the Buddha, “World Honored One, the amount of soil on a fingernail cannot be compared to that in the worlds all over the universe.”

“With this simile in mind, the Buddha preached to Kāśyapa, “Gentleman, it is as rare as the amount of soil on a fingernail for a man to be reborn as a man or for those in the three evil realms to be reborn in the human realm equipped with all six organs in the central land of Buddhism, and furthermore to have the correct faith, study the way of the Buddha, attain freedom by practicing the correct way, and then to enter Nirvana. On the contrary, it is as vast as an occurrence as the soil in the worlds throughout the universe for human beings to fall into the three evil realms after death, for those in the evil realms to be reborn in those evil realms again, and for those who were born to the human realm without having six organs properly functioning, to be reborn in remote corners where Buddhism is unknown, to believe in evil ideologies or to practice evil ways without ever attaining freedom or Nirvana.”

Many teachings are condensed in this passage, so some words of explanation are needed. The number of human beings who will be reborn in the human realm after death is as small as the amount of the soil on a fingernail while those who will fall into the three evil realms are as vast as the soil in the worlds all over the universe. The number of those in the three evil realms who will be born after death in the human realm is as little as the amount of soil on a fingernail while those who will be reborn in the three evil realms after death are as immeasurable as the soil in all the worlds of the universe. Those who will be born human beings are as numerous as the soil in the worlds of the universe, but those who will be born human beings with six senses functioning properly are as small in number as the soil on a fingernail. Of those born human with six senses properly functioning, those born in remote areas are as numerous as the soil in the worlds all over the universe but those born in the center of the land are as small in number as the soil on a fingernail. Even though those born in the center of the land are as numerous as the soil in the worlds all over the universe, those who encounter Buddhism are as minute as the soil on a fingernail.

Shugo Kokka-ron, Treatise on Protecting the Nation, Writings of Nichiren Shōnin, Doctrine 1, Pages 53-54