This explanation is from Master Hsuan Hua‘s commentary on the Medicine Master Sūtra.

When the Buddha was about to enter Nirvāṇa, Ānanda was so overcome with grief that he could only cry. Although Ananda was a third-stage Arhat, he was still emotional. He couldn’t bear to think that the Buddha was going to enter Nirvana, so he wept piteously, forgetting about everything else.

Then another bhikṣu (Venerable Aniruddha) reminded him, “You’re in charge of remembering the Dharma spoken by the Buddha. The Buddha is about to enter Nirvāṇa, so you’d better think things over clearly. There are some important matters you should ask the Buddha about before he enters Nirvāṇa. All you know how to do is cry! What will become of us in the future?”

Hearing the Venerable One’s words, Ānanda collected his wits and said, “Yes, you’re right, but I’ve been crying so hard I can’t think straight. What should we ask the Buddha?”

The Venerable One said, “First of all, when we compile the sūtras in the future, how should they begin?”

Ananda replied, “Right! That’s very important.”

“Second, when the Buddha is in the world, we take the Buddha as our teacher. After the Buddha enters Nirvāṇa, whom should we take as our teacher?”

Ānanda said, “Yes, that’s also an important question.”

“Third, when the Buddha is in the world, all the bhikṣus dwell with the Buddha. After the Buddha enters Nirvāṇa, with whom should we dwell?”

“That’s certainly a good question.” Said Ānanda.

“Fourth, when the Buddha is in the world, he can subdue the evil-natured bhikṣus. After the Buddha enters Nirvana, who should discipline them?”

“These four questions are all extremely important,” said Ananda. “Now I will go and ask the Buddha.”

Then he went before the Buddha, knelt, placed his palms together, and said, “World-Honored One, since you are about to enter Nirvāṇa, there are some important questions I would like to ask. I hope the Buddha will compassionately answer them.”

The Buddha replied, “What are your questions? You may ask them now.”

“Buddha, you have spoken the Dharma for forty-nine years and expounded the sūtras in over three hundred assemblies,” said Ananda, “In the future, when we compile the sūtras, how should they begin?”

The Buddha told Ānanda, “Our sūtras are different from the scriptures of other religions, which begin by speaking of either existence or non-existence. You should begin the Buddhist sūtras with the four words, ‘Thus I have heard’ which means, ‘I, Ānanda, personally heard this Dharma, which is thus; it is not hearsay.”

Ananda said, “Okay, I will use the four words, “Thus I have heard.’ My second question is, when the Buddha is in the world, we bhikṣus take the Buddha as our teacher. Whom should we take as our teacher after the Buddha enters Nirvāṇa?”

The Buddha said, “After I enter Nirvāṇa, you bhikṣus should take the precepts as your teacher. The Pratimokṣa is your great teacher. If you uphold the precepts, it will be the same as when I am In the world. You should avoid all evil and practice all good deeds.” …

“Now I’ll ask the third question,” continued Ānanda. “When the Buddha is in the world, we dwell with the Buddha. We always live and study with the Buddha. After the Buddha enters Nirvāṇa, with whom should the bhikṣus dwell?”

The Buddha answered, “After I enter Nirvāṇa, all the bhikṣus should dwell in the Four Applications of Mindfulness.”

The Four Applications of Mindfulness

1. Contemplating the body as impure.
2. Contemplating feelings as suffering.
3. Contemplating thoughts as impermanent.
4. Contemplating dharmas as being without self.

The first application of mindfulness is to contemplate the body as impure. “But,” you say, “I take a bath and keep my body clean every day, and I put on makeup and jewelry to make it beautiful.” You may adorn your body with expensive jewelry and designer clothes, but it’s just like decorating a toilet; no matter how beautiful the toilet looks, it will still stink. …

Second, contemplate feelings as suffering. Feelings refer to sensations. No matter how enjoyable the sensation is, it is basically suffering. Happiness is the cause of suffering.

Third, contemplate thoughts as impermanent. Thoughts arise in continuous succession, one after another; they don’t last.

Fourth, contemplate dharmas as being without self. All dharmas (phenomena) are free of the notion of “me and mine.”

These are the Four Applications of Mindfulness. …

“Here is my fourth question,” said Ānanda. “When the Buddha is in the world, the Buddha can subdue the recalcitrant bhikṣus. After the Buddha enters Nirvāṇa, how should we deal with them?”

The Buddha said, “When you encounter a recalcitrant bhikṣu, just ignore him – don’t talk to him.” To ignore him is a passive way of expelling him. If no one pays attention to him or argues with him, he will soon grow bored of making trouble. If you pay attention to him or try to fight with him, he will think that he is getting somewhere. But if you ignore him, he won’t be able to do anything.  …

Recalcitrant bhikṣus are perverse and unreasonable. The more you argue with them, the more they enjoy it. That’s why the Buddha instructed us to ignore them.

The Three Doubts of the Assembly

When Ananda first ascended the Dharma seat – the seat where the Buddha spoke the Dharma – to begin the compilation of sūtras, there were many auspicious portents. For instance, Ananda became endowed with the thirty-two features and eighty subtle characteristics of a Buddha. Seeing those portents, the members of the assembly immediately had three kinds of doubts:

1. They thought that Ananda was their teacher, Śākyamuni Buddha, who had come back to life.
2. They thought that Ananda was a Buddha from another world. “Is this a Buddha who has come from another world to speak the Dharma in the Sahā world? Otherwise, why would he look like that?”
3. They thought Ānanda himself had become a Buddha.

The entire assembly was startled and at a loss, but once Ānanda said, “Thus I have heard,” their three doubts were dispelled. He was in effect stating, “This is the Dharma that I, Ānanda, personally heard Śākyamuni Buddha speak.”

Even the Arhats had three doubts when they saw Ānanda leading the compilation of the sūtras. As we listen to this sūtra, we may have a thousand or even ten thousand doubts. “Is this sūtra true? Did the Buddha really say this? What proof is there?” Our human brain starts plugging away, generating millions of doubts. So, you see, studying the Buddha-dharma is not that easy. Why haven’t we had any attainment? Too many doubts, that’s why!

A cultivator should take care not to doubt.
Once you doubt, you go astray